You assume too much. I am very comfortable with separate regulations vis a vis public versus private spaces. Alcohol is, in most jurisdictions, illegal to consume in public, and being intoxicated in public is also, generally, illegal.
Given that the majority of occupants of college dormitories are, if not minors, than at least then under the legal drinking age, prohibition of the use of intoxicants is expected. That doesn't even take into account that dorms are college property, and subject to its regulation.
Sorry for not elaborating on the effect of legalization on college dormitiories; your fixation on them is cute, but it's been 25 years since I've even been in one so they're not the first thing I consider when reflecting on public policy.