The only way I can see them getting away with it legally is if they have a clear sign posted in front that says no food and drinks so they can force you to spend a bunch of money on your own snacks. Do theaters really have any legal authority to stop you from bringing your own snacks? or do these laws vary by state? I'm not sure how to research this.
Within the past year or two I've been told a couple times that I can't bring my own snacks or drinks and other times I get away with it, it's not like I bring anything weird like leftovers, I just bring my own soda and candy I buy from winco.