I'm going to leave trash in a movie theater
Why would you leave your trash?
It is a way of showing them I don't care about their rules.
You're a guest in their theater regardless if you paid for a ticket. They put trash cans out so that you can throw away your own trash.
I think I'll throw away my popcorn cob when the movie ends anyway.
Thats fine if you no longer want your cob, as long as you dispose of it properly. Movie theater workers aren't paid much and cleaning up after one self is a basic obligation that every adult should learn to do.