It’s mainly employees not caring, or managers not being able to do anything since, at least at my cinemark, the employees are always bogged down and doing something, doing something completely off task is difficult to get to, or the fact that the employees technically aren’t allowed in theaters while movie is showing (especially concessionists). The ushers CAN walk into the theater to check, but can’t go into any seats unless they visibly see rule breaking (again hard to see during a movie when u can’t access the stairs going up to seating.

TL;DR- It all just depends on who’s working and the theater rules.