""They probably don't want ice cream dripping all over the theaters? Did you really need to ask this?"
they already sell popcorn and chips and soda and candy and alcoholic beverages though"
On a practical level, cleaning up spilled chips, popcorn, and candy are much easier to clean up then something like ice cream. Think about a movie theater--if you go in to see a movie, can you tell if someone dropped popcorn? Maybe, but most likely it's been swept up somewhere. Now think about the spot where someone spilled their soda, slushy, ice cream, banana flavored frozen daiquiri, or any other sweet thing. Yep. You're thinking about the noise your shoe makes when you step in it. And that's after it's been cleaned up!
If theaters want to sell food and drink, awesome. That's their choice. I think it needs to be restricted to areas outside the main auditorium. Once you're about to be seated, it's water only--I'd rather hear someone open a bottle of water and take a sip then listen to them cough or be constantly clearing their throats.