Movie theaters don't make the bulk of their money from ticket sales. They make them from concession stands and profit margins are in the whopping 1000%+.
A small soda is $4.75 and a large one is $5.75. A small popcorn is like $5.00 and a large popcorn is like $5.75. Hot Dog, ICEE and nachos all $6 bucks or more. These 40 oz fountain drinks cost 15 cents in syrup and carbonated water and they buy bulk bags of kernel equating to again, less than 25 cents per bucket of popcorn. These are huge, huge margins and very little health/safety oversight is needed to serve these kinds of food so you can hire the high school kids with little training. Unlike McDonalds or Jack in the box, movie theaters don't need any advertising for their food. The movie itself is the advertisement. The increased traffic alone should help drive these sales up as people will have more money to spend.
Not sure why AMC is upset with that. If I was a movie theater owner, the more people I can get into the door, the more likely I can get more sales at the concession stand.