Theaters these days make most of their revenue from food.
There is no legal or statutory provision to prohibit one to carry personal food and water to cinema. This is a rule flouted by cinema halls having their own stalls for food and beverages. The reason, of course, is that the theatre wants people to buy their expensive food. Most theatres do not make money from ticket sales alone. They make money from selling food and drinks. As a result they don’t allow people to bring food and drinks in. The theater owners charge exuberantly for edible items sold inside. The way in which movie theatres conduct their food business is a form of racketeering, which is very much a crime. Buying something to eat in theatre will definitely burns a hole in the pocket. It is something like offering of dishonest service to solve a problem that wouldn’t exist if not for the actions of the entity offering.
A theatre is not a cinema that sells popcorn but it is a popcorn stand that shows cinemas. Money cannot be made by showing movies but a lot of money can be made by selling popcorn for a bucket at Rs.370-380 instead of Rs.25-30 a bucket. The theatre makes more money by selling popcorn and candy, so they try to establish a monopoly on movie snacks by banning outside food and drink.
The restrictions particularly affect old people and those who due to health issues cannot eat outside food. As long as one doesn’t leave wrappers and bottles behind for the cleaners to pick up, and as long as one respects and cleans-up after, it is okay to carry food into a theatre. One has to make sure that the food being carried in would not cause any unnecessary problems for other visitors or the staff. Food with thick stenches should be avoided.