Why are Las Vegas cabbies so weird about tips?

[BoyChick]

They might get squeezed on fare/tips, but they do get a pretty nice 'kick-back' from adult bussinesses like stripclubs, massage places and the like, when they bring customers there. Mind you, they demand this 20% whether they suggested the place OR it was your idea. I.e. if you jump in a cab at the Rio and say 'take me to the Chicken Ranch', they will make a few hundred bucks ADDITIONAL to your fare and tip, from the C.R. So maybe they prefer the customers that go to those places, and get 'angry' at people who don't, bc they make less money.