The trick is to start with yourself.
If you consider it a problem, then never do the seat chope-ing thing.
When your friends do that, tell them that you disagree with it.
I don't think it's a big deal if you sit down at someone's "choped" seat.
All you have to do when they get back and ask you why you are in the seat, is to say that
"Oh, I didn't know that putting a pack of tissue on the table means you've reserved the seat."
They won't be able to argue with you convincingly because they know that it's a social practise but it's not actually a proper way of doing things, they don't really have any legal justification against you.
It's the same thing for jaywalking: alot of people do it, doesn't mean that it's actually right and proper.