Re #3: Game theory suggests theaters can approach the same higher prices without colluding. Seeing as how price collusion is illegal (in the U.S.), and the results are reachable in another way, the accusation isn't necessary.
I'm curious what the effective difference to society is between a group of businesses all agreeing not to compete on price, and a group of businesses which each individually decides not to compete on price.
Maybe not necessary, but likely accurate. You can say that price collusion is illegal but until those laws are enforced that's not much of an argument one way or another.
Game theory assumes all agents are rational. They aren't. Pricing popcorn 20% below a competitor may seem like a good idea to a business owner and may indeed sway a consumer to return (people say things like "the ticket is more expensive, but I think the cheaper popcorn is worth it", even though numerically, it isn't)
If there is only some token competition on popcorn prices and there is research showing that that usually doesn't happen in real markets, then that is a pretty strong suggestion they are colluding.
