The theater house rule is that no outside food or drink is allowed, but clearly the only purpose of this rule is to support the brazen markup of food and drink at the theater's own concession stand.
I find it difficult to find any harm to the theater in eating smuggled popcorn rather than popcorn purchased on the premises, even when playing Devil's advocate as hard as I possibly can. My worst attempts even end up replacing 90% of the theater employees with a row of vending machines and a subway-style turnstile.
I have a friend who used to be a supervisor at movie theater, and he told me that ~80% of the theater's ticket proceeds went directly to the movie distributor. This is apparently normal in the movie industry.
If concessions went away, theaters would probably need to raise their ticket prices a lot. Even if you drastically cut the employees, there's still rent/taxes/insurance/cleaning. The last two could also conceivably go up in price if it became normal to bring in your own food.
economics of movie theatres: https://arstechnica.com/uncategorized/2006/01/5905-2/