Tip for Tat

What is with all the tip jars these days? It seems like everywhere I go there's a tip jar beaming guilt waves at me from the counter. I honestly don't understand. Why should I tip someone for simply working a cash register correctly? Aren't they being paid a full wage?

The place where I ate lunch yesterday has a tip jar by the register. I went up, ordered my food, and then came back to pick it up when they called my name. No one brought my food to my table or kept my drink filled for me; when I wanted more Diet Coke I had to go up and stand in line again and ask for some. So for what would I tip them? For handing it back without dumping it down my front? For filling it with actual Diet Coke instead of water from the sink?

Don't get me wrong--I've got nothing against them. They seem like nice folks, capable of providing me with a lovely baked potato without fuss. That's their job, though, right? It's nothing above and beyond the call or anything. They didn't run outside and wash my car while I stood in line for that second Diet Coke. I'm not trying to be a bitch about it (shut up) but I sincerely don't see why they feel they can expect extra from me for the privilege of standing in their line.

I never put anything in these pushy new jars. I don't like to feel manipulated. Am I the jerk here?