Essentially, theaters make as little as 20-30% of the total revenue from the movies they show, with the rest going to the studios, due to uneven financial negotiations (studios get around 90% of the revenue from a movie in its first week of release, which diminishes over subsequent weeks with more money going to the theaters). This causes cinemas to seek money from other areas, and concessions are the logical choice -- the studios can't touch them, and they can mark them up to the point where they're actually profitable. This price-gouging actually keeps theaters in business, believe it or not, since they don't make a lot from the movies themselves.
So when you see some $5 Junior Mints behind the counter while you're in line for Interstellar, know that there's a reason behind the price. And that maybe someone should complain to Ruth Bader Ginsburg about it.