To turn this away from the whole alleged racism, Boston isn't exactly the sexiest town. Think Paul Pierce would've chosen to get picked by the C's? Rondo? Baby?
The winters are harsh, the summers are wet, and the perception is that everyone there is white and Irish. That's the over-generalization.
If a player comes to Boston willingly, they come because they want to win, and that is a recent thing. History is cool and everything, and every top basketball player says they're a student of the game, but come on. Nobody wants to go to Boston because Bill Russell, Larry Bird, etc have played there.
The best thing the Celtics have going for them regarding FA's is excellence on the court. If you tell a millionaire "You can play for Boston and do mediocrly, or you can play for Miami and do mediocrly, I imagine the queue starts in South Beach.
Minnesota, Toronto, and Detroit have similar problems. Milwaukee's not getting Kevin Durant. If you're a cold city that isn't New York City, you're gonna need another gimmick besides history.
That said, I think Shaq, Sheed, Joe Smith, Jermaine, they're helping to change the paradigm.
You notice that Boston hasn't failed to resign a major free agent in what..20 years? Show them they can win, show them a team that has a locker room that goes about its business, and people want to stay.