We also asked them to book a table for lunch at the restaurant in Epcots Mexico. Mind you they booked it for 1am and not 1pm so don't feel too bad about that one!
I think the argument is that if you leave less than that you are actually costing the server money not just leaving then nothing as they are taxed on tips they are estimated to receive and have to pay out a portion to bus boys etc.
On a more positive note in 12 plus trips to US only had appalling service once - DH spoke to manager and whole meal was comped.