Holy false equivalency, Batman.
Cheering in a theater isn't rude unless you are loud the entire movie. Being excited at an exciting moment is fine. Do you also not laugh out loud at jokes? If you do, YOU ASSHOLE!
It's not a false equivalency. Not remotely. I am showing you that the things some people find fun can be unpleasant to others.
Cheering in a theater is always rude even if you only do it for a second. Being excited at an exciting moment is fine. You don't need to hoot and holler because you're excited. Laughter is much harder to hide and I think we all expect people to laugh at funny parts in movies. Everyone who's not 15 should be well aware that screaming during a movie because you think it's cool is not acceptable.