It is so it will not break and make your clothes reek of alcohol. I suggest wrapping the bottle in bubble wrap and then put it inside a LARGE (2 or 3 gallon ziploc type of bag)....you can then wrap it in a piece of clothing (but I do skip that part of it.)
I have never heard of anyone on Princess who has liquor in their CHECKED luggage confiscated. Just be sure to take enough with you on embarkation as they will not let you buy on board and take to the stateroom until the last day of the cruise....and depending on what ports you go to, they MAY not let you bring any liquor on board without confiscating it and delivering it to you the last night of the cruise.