My usual movie theater recently had a high school age kid staffing the ticket booth when you walk in and a brand new sign saying "no outside food or drinks." I used to just carry by hand a water bottle and some snacks or put it in an old grocery bag. This recent visit, I was told outright "no, that's not allowed, leave it in your car."
I was really surprised, been going to this theater for probably 5+ years now and this was the first time they'd complained like this (maybe the kid just felt he had to enforce the rules more than usual, not sure).
So I just went back to my car, took everything out of my bag, stuffed it all under my shirt/into my pockets like I used to 10-15 years ago, walked back in. No complaints.
Honestly 80% of the time, anything I bring in will beat whatever they sell anyway.
And FWIW, the last time I bought a bottle of water there for $7 or whatever, I found out during the movie it had already been opened/had a broken seal, so it was a waste of money.
Meanwhile this last time I brought in a bottle of water I bought at the store for maybe a dollar, sealed, etc. No problem. Shrug.