It's not okay with them. I guess that's what matters if you want to be able to come back.
I usually eat well beforehand and then just order a tall drink as I get thirsty no matter what. Sometimes sneak old holiday candy in my purse but nothing big (and don't throw garbage on the floor). When I was in high school we'd sneak a 12-pack of cheap Taco Bell tacos in and share amongst friends (that sort of sucked...but sure was good). The boss was our friend... (:
At drive-in theaters, which are rare, the food is so much cheaper but the movies are usually not first release and the crew working the theater is very small. Usually better food too. I'm sure it used to be more like that once mom & pop theaters were more common post-trust-busting of theater/production company corruption (where movie-making companies owned their own theaters as well once upon a time). Now the theaters are all part of big companies like Regal, Century, etc.
People all want to see extremely expensive blockbuster films with big stars, so we get tons of ads and big ticket prices. The theater's only real money after paying to show the film is from concessions...so you are paying for something worthwhile (somebody's job and a theater to go to). If people wanted to see more modest or artistically beautiful films in the theaters the costs might (and I mean might) go down. If you want the big movie palace and want the big royalty films, then you are are getting what you want to pay for--even if it sucks.