I have a really good friend that bartends at the Fridays in DFW. Makes great money. 
Of that, I am quite sure. Tips are a % of the bill, so if it is expensive, so the tips will be up. And travelers are always grateful for a sympathetic ear, or headed for vacation and are getting an early start on the liquid fun, and many have expense accounts that will pay the tab, so there are generous tips.
Not that they don't deserve it. I have greatly enjoyed layovers with good humored bartenders -- it makes a difference.