Tipping has been studied and has been associated with discrimination on gender, race, and class.
http://www.esquire.com/food-drink/restaurants/a23132/why-tip...
http://www.esquire.com/food-drink/restaurants/a23132/why-tip...
When they were sued by their own workers because of this and had their backs against the wall, they had to claim it was because of this racist hand-wavy BS.
It was a ret-con and an incredibly cynical and insulting thing to do, but that's now the official reason they didn't allow tips.