To add a different kin of unpopular opinion:
I hate the whole popcorn munching in cinemas, so I applaud everyone who sneaks in their own stuff, as that won’t be popcorn.

Besides that, cinema halls are practically never full, so the cinema earns practically 100% of my entry price without loosing anything (as the seat I sit on doesn’t care if it is occupied or empty).

I never buy snacks, but I also don’t sneak anything in.
I don’t understand the whole obsession with eating tons of stuff while watching a film in public. I think that is a really strange urge that many seem to have.