I disagree. When you're on a bus for a long time and the option is either to sit with whatever trash you have on your lap/put it in your bag etc, or drop it, you may well drop it. If there's a trash can conveniently located, most people would place their stuff in it.

It's not foolproof. There are people who toss trash around regardless. It's a question of making people go out of their way to do something vs allowing them to do it easily.