This is someone who gets paid $2.63 an hour and lives off of tips. They intend to be tipped at a certain rate (oh yeah, it's 20% by the way. If a drink costs $12, one dollar is a bad tip, even $2 isn't very good. But that's another blog for another day). Often selling you that salmon fillet and and ceasar salad with no croutons and dressing on the side all boxed up and bagged is just as much work as making a few drinks, or serving you that identical dinner at the bar. Because of this the bartender believes he/she should be tipped in the same manner. If you don't they are going to curse you out and call you names to their coworkers as soon as you leave, regardless of how friendly you are, or how many times you say "thank you." It's just the way it is. Please act accordingly.
P.S. Most decent restaurants have a position called "food runner" who gets paid by earning a portion of the tips that the waiters/waitresses and bartenders make. Often the amount is determined by a percentage of the total "food sales," not a percentage of the actual tips made. In those cases, if you increase their food sales without tipping the bartender is actually losing money in order to have the privilege of doing business with you.
For lots of other commentary on restaurant and bar behavior, click here. Or on the tab labelled "bar stuff" in the topics on the right.
2/24 Update: If you have an opinion on this I definitely recommend reading the comments below.