[Havoc]

Mostly because the picture you posted (or the picture I saw, which I assume is what you're talking about) falls in the same category as some rage comic shooting some person everyone hates. Or someone blowing up someone's house because they made a stupid remark on Facebook. It's horrific if someone would actually do it, but everyone assumes and pretty much knows it won't happen and that it's just internet humor.

If some guy posts pictures of himself with home made pipe bombs, a table full of weapons and names a date, location and event then of course you should take it seriously. But a picture of a movie theater with some random text on it hardly qualifies as a ligit threat. I've seen stuff more harsh on 9gag that was just meant as humor and everyone knows that.

If a cybercrime unit would spend time on every single picture or tweet that might just be some vague threat, they would be pulling double shifts around the clock because the internet is just fucked up that way.