Why is it that there is now a tip jar absolutely everywhere you go?

I don't get it...I thought the system of tipping existed to reward (and encourage) good service.  But these days even fast-food burrito shops where you bus your own table have tip jars at the counter.  Help me out here...if I've already paid for the food, I have to pick it up from the counter, and you want me to clear my own table, what service is it you're providing, exactly?

My penchant for vanilla lattes may border on an addiction, but dammit, if Starbuck's is going to charge me three-and-a-half bucks a cup, they'd better fix it exactly the way I want it (grande nonfat extra foamy, thank you) without expecting me to shell out even more for a tip.  For that matter, at three-and-a-half bucks a cup, the barristas better be getting a big enough cut from the corporate mother ship that any tips they do take in are just gravy, not the better half of a paycheck.

I like to think I'm pretty reasonable about tipping...assuming the service was anything better than absolutely terrible, I usually tip 15% for breakfast or lunch, and 20% for dinner, more if the place is inexpensive.  I sympathize with people who are just trying to get by, and who are willing to bust their hump to make my day a little better.  But there's a difference between going above and beyond and just doing your job.  Next thing you know, they'll be expecting tips at McDonald's.

Yeah, that may be bread in Billy Joel's jar, but aren't you getting paid to do this?