Cinemas compete with each other for business, but for most of their customers, the price of popcorn is not the most determining factor when they choose which cinema to go to.
Imagine yourself planning an evening at the cinema, and trying to decide where to go. Most people would consider the following factors:
- Does it show the movie that I most want to see?
- Does it show that move at a time which is convenient to me?
- Is getting there convenient to me? (Transportation, parking, etc.)
- Is the quality of the theatre itself good (screen size, sound, comfortable seats, etc.)
- Is the prize of the ticket itself reasonable?
I would venture to say that most people do not consider the price of popcorn or other snacks very high (if at all) on this list. Most movie theaters do not even list the price of popcorn on their websites, so often you would have to call them to even find out.
Once you have decided on a cinema, traveled there, bought your ticket, etc., it is too late then to change your mind and go somwhere else. Some people may elect not to buy popcorn at all because of the prize, but those people are fewer than the people who do buy the expensive popcorn, therefore it is profitable for the cinema to sell the popcorn at an expensive prize.
If the popcorn is bizzarely expensive, you may remember it for next time, and be less inclined to revisit. However, the prize of popcorn may be sufficiently far down on people's list of priorities, that they may even ignore it, if the other points mentioned earlier suit their needs. Also, the price of popcorn does not vary very considerably between cinemas, so this does not really happen anyway.
Imagine that a cinema, in an attempt to attract more customers, lowered its price of popcorn down to market value. I do not believe that customers choose their cinema based on popcorn price, so therefore I believe this would fail to result in a considerable increase in business. Thus, the cinema would start to lose revenue from the decrease in popcorn earnings. They would then either have to explain this to their owners/shareholders, cut costs somwhere, or hike their ticket prices (which likely would affect ticket sales, although negatively.) Therefore, the wisest thing to do, from the cinema's point of view, is to keep the popcorn expensive.