They let you bring Wine on board and will charge you a "corkage fee" to use it in the dinning room.. They should let you bring your gin on board and charge a similar fee in my book. There is either some legal reason they don't or they just have not considered this avenue. Maybe they figure that there are not enough people willing to pay extra to be able to make their own cocktails in there cabins?
Of course, I'm not normally in the cabin long enough to worry about not having a drink. And there is certainly no shortages of opportunities to find a drink outside of your cabin... But I understand the desire.
Carnival Cruise Lines:
Fantasy 90, Celebration 95, Victory 05, Conquest 05, Spirit 6/06, Freedom 6/08, Holiday 7/08, Victory 6/09, Valor 9/09, Dream 6/10, Splendor 8/10,Conquest 8/11,Magic 11/11,Triumph 4/12,Spirit 7/12,Freedom 11/12, Triumph 2/13 - The fire cruise
NCL Dream October 2006, RCCL Mariner OTS June 2007, Celebrity Silhouette 2013