Here's a really stupid argument:
1. Term X can be used in a racist way.
2. Other uses of Term X are therefore racist.
It's got to be one of the poorest excuses to call someone a racist I've ever seen. Yet people insist on doing it to unsuspecting politicians or other public figures. It's for this reason that Governor Mitt Romney of Massachussetts has been bamboozled into apologizing for an action that is in no way wrong. Tony Snow has also been criticized for using the same expression in its original, non-racial sense. A tar baby is generally a sticky situation, and nothing about race is implied by this use of the term. It's origins come from an African folk tale, and its function in accounts of sticky situations has continued undisturbed by those who ignorantly coopted it for racist purposes. In the northeast, where Romney is governor, most people have probably never even heard of the racist use of the expression, and those who do encounter it might easily forget it as so far out of their vocabulary that it doesn't enter long-term memory.
Another example I've encountered (in this case only very recently) is "call a spade a spade", which simply means to identify something for what it is. Some racists in the South have apparently called black people spades as a derogatory term. Since I've never hung out with those people, it never would have occurred to me that someone would do so. Why should an uncommon use of a term in a localized region, a use I've never even heard of, make my use of a perfectly normal idiom somehow immoral? Those who treat such statements as racist seem to me to be linguistically unaware at best and incapable of moral reasoning at worst.
It's completely unreasonable to expect those who did not grow up around racists to know everything that racists might say. So why should I be expected to know of some racist use of terms like 'tar baby' and 'spade' before I might happen to use some fairly standard figures of speech involving such terms? For the same reason that we do not expect a very small child to have committed a moral mistake by using terms that are unambiguously racist such as the N-word, we should not expect adults to know the racist meaning of a localized use of an expression that their linguistic community simply does not repeat.
Some might protest that everyone should be more aware of racial issues, and certainly that's true. But what racial issues should people be more aware of? The culture of offense that generates how Romney is being treated will encourage people to spend their time learning about racist speech in the past rather than encouraging them to identify real problems now that they can do something about. So it's a little unproductive. But that's only a minor complaint. The real problem is that it doesn't lead white people to spend time with black people, learning to appreciate them and interact with them on levels they may not have interacted with them before. It instead encourages them not to seek friendship but to guard themselves defensively from charges of racism, to research every possible way that innocent words might offend someone to stave off the attack. It thus causes white people to see black people as the enemy.
At this point in U.S. history, the charge of racism is one of the most damaging attacks anyone can make about anyone else's character. Those who so blithely throw it around this way are generating anger, confusion, and fear among those who would otherwise perhaps be spending their emotional energy in racially productive ways. That consequence is not just unfortunate. It creates further unfortunate consequences and prevents plenty of good consequences in the future by preempting a lot of good will that would otherwise be present.
So much for the case of someone who doesn't know of the baggage some people bring to their hearing of such terms. What about those who are aware of such racist uses? Surely they do know that it can be used in that way, so isn't that a reason to be careful and not cause offense? I don't think so. After all, those who thought 'niggardly' was a racial epithet were the ones making the mistake. No obligation falls on those who know that it is etymologically unconnected to the N-word and who know that its meaning is simply "stingy". The word has never meant anything else and continues to be a perfectly good word for what it's a word for. If people wrongly take it to be offensive, then it's offensive only in the sense that people actually do get offended, not in the sense that it itself is a legitimate cause of offense. No one ought to be offended by it, and those who are have made a mistake, one that gets compounded into a moral mistake if they pursue it even after learning what the word really means. So the mere fact that something might offend, even if I am aware of such potential offense, is not a reason for not using the word.
Is there something special about this case, then, given its actual history? The 'niggardly' case doesn't have that kind of history, nor does the case of 'picnic' (despite some claims to the contrary). Ultimately, this question depends on whether the racist use of 'tar baby' is the predominant and expected meaning in the context. Governor Romney was talking about a construction project as a political issue he doesn't really want to touch. Nothing in the context suggests anything racial. It looks to be exactly what he says it was, a sticky situation. Thus his use of the term hails back to the classic African origins of the expression and not the racist co-opting of it by ignorant white people whose efforts to change the word's meaning ought to be resisted rather than encouraged.
The original and most common use of an expression should be taken as the dominant, expected meaning whenever the context does not show another use to be more likely in that case. This is just a general principle of interpreting other people's statements. It is so with or without the existence of some deviant, racist use of an expression. Anyone who thinks otherwise is simply incompetent in the use of that term, and the fault is in the reception (whether a moral fault or just a linguistic one; I won't take a stand on that). In that way this is just like 'niggardly' except that in that case there was no racist use except in the ignorant imagination of the offended party who didn't know what the word meant. In this case there is a negative use, but it's not the standard meaning and therefore should never be our assumption as to a speaker's intent unless the context indicates otherwise.
Now if an expression's racist use gets so common, and its original use so uncommon, that the racist use gets assumed, then I think the situation is very different. I don't that's even close to the case with 'tar baby' or with 'call a spade a spade'. My complete lack of any awareness of the racist uses of those expressions demonstrates that. It's not even part of the linguistic toolbox of many speakers of U.S. English alone. So I think those who bludgeoned Romney into apologizing should apologize to him and take back any criticism they might have offered him. They should publicly condemn their suggestion that there was anything racially insensitive about his comments and ask his forgiveness for tarring his name in such a morally loaded way. The charge of racism is far too serious to throw around like this. It belittles real racism to do otherwise.