As wonderful as movie theater popcorn can be, this seems to be an extreme reaction to not getting it.
Of course, taking an empty popcorn tub out of the trash is also an extreme solution to the problem of wanting free popcorn in the first place.
While movie theater snacks are certainly expensive compared to their cost outside the theater, concessions is where theaters make their money after all, popcorn isn’t usually all that expensive.
They may have just told him they were closed to try and get him to go away, not realizing how he was going to react.