There are a couple of arguments to prevent discrimination against riders. The current regulation of Taxis and Public Transit are such that disabled persons and wheelchair-riders can get rides about as easily as able-bodied people. Uber and Lyft have no such provisions.

On the flip-side, Uber seems to be reducing racial discrimination against potential riders, as per this piece: https://medium.com/matter/ubering-while-black-146db581b9db.