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In a competitive market, why is movie theater popcorn expensive?
To add to Jason's answer; a further mechanism is that of monopoly rents which you mention in your question.  Movie theatres are often in shopping complexes (which themselves may offer a particular cinema exclusivity), or physically remote from each other, making price comparison more difficult.  Different companies may not offer the same movies (similar to the way phone companies offer difficult-to-compare contract pricing). Once you've paid for your movie ticket, if you're suddenly thirsty or peckish, the theatre is the only place selling snacks.  Many theatres (including film theatres) discourage (or refuse) patrons from consuming products purchased elsewhere on site. A sense of "capture" is reinforced with ticket collection at the entrance or some form of barrier (inside vs outside the cordon). A theatre can thus capture their patrons and then leverage that access in order to discriminate amongst the higher-paying consumers mentioned by Jason.