The idea that Boston isn't a particularly racist US city and the idea that the city of Boston might be a bit more racist than some other places are two separate things that should be considered as such. The first can be true while the second can be false (although in a perfect world we have moved on as much as some posters might like to think we have).
Having said that: the city was certainly, if tacitly, segregated while I lived there -- and I'd imagine that even with the slow creep of gentrification that there aren't yet a lot of white folks flocking to live in Roxbury. Not a situation unique to Boston in the slightest, but the fact is that it is one of those smaller cities that simply doesn't seem to feature an awful lot of racial co-mingling compared to a lot of other cities around the world, or even around the States, especially north of the Mason Dixon. Factor in the rest of New England being about as ethnic as mayonnaise and you've got a recipe for homogeneity among the fan base that isn't going to be unnoticed.
Put more simply:
I don't think Scal becomes the same sort of celebrity playing for the Nets for his entire career, even if you swapped the trajectories of the two teams over his relative tenures with them, and even though we are all sure that he wouldn't have wanted the mantled foisted on him if he had a say, Larry Bird was definitely stuck as the white saviour for a whole generation of bigoted NBA fans who are mostly still alive and kicking. The Kevin Love thing has already been remarked upon. The Hayward thing is obviously cut from the same mold, which is a perception-slash-caricature that, however unfair, has stuck around the Celtics.
Which, obviously, sucks.