At this point, we really don't know what "that" is.
It could be just taking a stack of posters off a pile (no damage) or off a wall (minor damage). It could also be breaking display cases in order to get at the posters or other items inside (serious damage), or breaking actual displays and memorabilia (major damage).
At which point is it no longer funny?
There are some places where "funny" doesn't exist. Cerek mentioned bomb threats. Crying "Fire!" in a crowded place falls into the same category.
I've related the story before of a friend of mine going across customs and when asked if he had anything to declare, queried another friend to ask if he thought that included the M-16 broken down in the trunk.
Customs was not amused, and went looking for the M-16. And they can disassemble your vehicle while doing so without a second thought.
I fly a lot... and inside (and outside) the US, joking doesn't exist when going through security. If you don't believe, me, joke that you've got a bomb with you. They'll find it really funny
Now, stealing and vandalism in a movie theater are nowhere near the same level as crossing borders or getting into airplanes. I just offer the latter as proof that "funny" has limits.