I were in US a couple years ago and found it pretty emberrasing how people are expecting a tip. I understand that they're paid less and i've spoken with some of them and waitresses etc may make about $3/h+tip. It's not the customers job to see to it that the employees stay at their job - it's the companies. At the hotel bar we had some kind of happy hour where everything was free which of course made it a bit crowded so unless you held up a tip / bribe the bartender just ignored you.
I understand that there are cultural differences but in my opinion you get a tip for a job well done - but it's ALWAYS optional and you should never feel obligated to give one.