Two reasons:
1) There is so much crying wolf from the Al Sharptons and Jesse Jacksons of the world that it diminishes the true incidents of racism that do still happen in this country.
2) It is such an inflamatory accusation that to accuse one of being a racist when it is not the case is one of the more damaging things you can do to a person. In addition, once the accusation is out there, it is difficult to shed it. You may very well continue to be perceived in this way even if untrue.
My own opinion is that, in many cases, black people use this term to describe behavior because they either assume all whites are racist or because they just react. They accuse without thinking. They "play the race card". Let me give you three examples:
1) I will not be voting for Barack Obama. I am white. He is half white, half African. In a recent discussion at the barber shop where I go to get a haircut, politics comes up. When asked about Obama by a black guy in the chair, I respond "I would never vote for him". Instead of asking me "why not?" He calls me a cracker. I tell him that I grew up in Chicago. Then he calls me a racist. I explained that I would be delighted to vote for a black man who shares my values but that Obama does not. In fact, he is opposite my values on just about everything. He is for higher taxes, bigger govt, pro-abortion, negotiating with Iran, etc. He is a marxist with absolutely no experience and, in my opinion would be terrible for this country if elected president. But, the guy in the barber shop never asked me about any of that. He just assumed I am anti-Obama so I am anti-black. How narrow-minded.
2) you remember that New Yorker Magazine caricature last week? Did you see all the black responses to it? Absolutely hilarious! The New Yorker is about as far left a publication as you can get. They are very pro-Obama. This was satire. They were poking fun at conservatives who call Obama's religion and patriotism into question. Yet, black people come out and slam them saying "it just goes to show that some American's arent ready for a black man to be president". What? Did you think before you open your mouth and cry "racism"!? Obviously not.
3) The "N" word. Again, this is back in the news. Some idiot on "the View" says, "its OK for black people to say this because when we say it means ______. But, I do not wanna hear that word come out your mouth or any white person's mouth". And Jesse and Al say whites have a double standard? This sort of thing makes me want to vomit. In fact, it is this very behavior that fuels alot of the real racism against blacks in this country.