They probably "get away with it" because nobody has ever questioned the legality of the practice, in the manner you suggest. But I wouldn't be too hard on the movie theaters. I worked at a multiplex in my youth, became friends with the higher-ups and learned a bit about how they work. When you pay $10 for a movie ticket, especially to see a first run movie, the theater operator receives a very small percentage of the ticket cost. Most of it goes to the distributor, producers and to Robert Downey Jr. The costs of running the theater, like any business, include payroll, utilities, rent/mortgage, taxes, etc., and the pittance they keep from your ticket price hardly covers those costs. That is why the concessions are so expensive, and perhaps why it is necessary to sell some advertising, to generate another income stream.