It's simple, no one wanted to be arrested for assault and battery, and maybe even charged with a race crime. They may not have been found guilty of the crime, but it is highly likely everyone involved would've been hauled downtown and booked until trial time, especially if there'd have been more than one defender beating down the assailant.

This way, none of the standers by got cuffed and stuffed. Maybe not too moral, but they aren't under any legal obligation to intervene in any way.

This is the type of society we have elected to live in. I don't like it much either, but it is what the majority seem to want, and it's still the best deal going, so I guess we make the best of it, and get our licks in when we can.