Movie theaters are private businesses (like the whole of the music and film industry). If a private business sells something, the conditions of the sale should be (negotiated and) agreed upon between seller and buyer beforehand, e.g. by the seller handing out his (general) conditions on paper before he sells.

I think it's reasonable if there's laws protecting businesses and consumers in cases where conditions were lacking or outrages.

A law that would allow consumers to end up in jail for filming in a theater, even without proof that the filmed material has been spread, is, let me say 'heavily biased favoring the movie and music industry' instead of straight out 'unreasonably unfair and ridiculous'.