I would love to see someone bring a lawsuit against a cruise line in Europe on this matter. I suspect that the European Court of Human Rights would find that the cruise line was guilty of denying the cruiser his basic rights. I have never heard of a hotel that denies a guest the right to bring a bottle into his room and drink whatever he wishes as long as he doesn't bother anyone else. I'm only talking of in-cabin drinking and not bars. I think the same would apply to cruise lines.
I have found that cruise lines don't usually run the security and embarkation ports so booze can be brought on the ship. Of course, this doesn't apply at stops along the way.