I think one of the reasons people tip is in this country, we really seem to hate coins, especially non-quarters. I find loose change, especially pennies, everywhere. Left in bins at check-points, on top of tables after people finish quick meals etc. When I worked for Enterprise, I'd find coins in the cupholder in at least 25% of the returned rentals.

The cups are there, people think "I don't want these coins anyway" and dump them in. Personally, I don't tip because I like carrying change, it's how I finance my weekly Powerball habit