If you really want to concentrate just on your question rather than the wider debate then Racists think that they can get away with it because most of the time, with what I would call “low level” racism, they do. So do many burglars, drunk drivers, and domestic violence perpetrators. In fact, almost everybody who commits a criminal offence believes they will get away with it and many of them do. Mr Sterling's experience as a victim is not exceptional.
All sorts of people suffer all sorts of verbal abuse day in day out from the mouths of people who are simply too badly mannered, badly behaved and ignorant to know any better. But when push comes to shove the police cannot deal with all such complaints. Being abused because one is fat or has ginger hair is just as distressing to the victims as being racially abused is. They are abused for their physical attributes which, by and large, they can do nothing about. Racial abuse has a special significance because it has been subject to very high profile debate and legislation but the effect on the victim is no more or less profound than any other similarly targeted group. But the others don’t have a trump card to play and are not usually on the telly when the incident happens.