Right. I've read these disclaimers as well. And while they may well confiscate openly flaunted shore-bought beverages- after all, drinks aboard are a major profit center for the cruise lines. On embarkation day- what THEY call 'CHANGEOVER' when the 'old' passengers (and their luggage) disembark from about 7 A.M. to about 9:30, then the stewards have to clean and put the cabin back to 'postcard' condition before the 'new' passengers come aboard from about noon 'til maybe 4:00 P.M., then the ship sails about 5. The cabin stewards have to meet their new passengers and deliver 3,000 pieces of luggage by about 7 P.M.- NOBODY has time to be digging through locked luggage for something that will only pi$$ off the passengers and not net them, the employee, anything but hard feelings.
Yes, they CAN remove brought-aboard beverages, but in the real world, it's highly unlikely to happen. They may publish stuff like the above, but in reality, a repeat passenger is worth his weight in Jack Daniels, if not gold.