Sure is their job, so is the server that brings me a breakfast every now and then. I tip min 50% of my bill to all diner servers. My mom said it was one of the hardest jobs she had while going to college, and said the tips were horrible.
Separate thought:
Some people just do things because they can or want to, as long as one is not being obnoxious or looking for a thanks I see nothing wrong with it. People that do have a problem with what others do with their money. I just don't get that. Spend it, give it away, hoard it. Who cares.