I wonder if there was a male passenger trying to book a room by himself, with no tie-in to any other travelers, if that would trigger some sort of red flag. I know that at our local Museum Center in downtown Cincinnati, they will not let any unaccompanied males enter the Children's Museum.
I hope they never do this - as I fit into the category of solo male traveler. I have cruised solo, with family, and friends - if I was ever questioned about taking a cruise on my own for ill intentions I would be extremely embarrassed (if it was DCL or another cruise line).
Second, from personal experience, Customs and Border Protection (CBP) is VERY through in it's check of passengers - before, during and after the cruise. If someone was sailing solo (or with a group) and had a conviction CBP would most likely question that person before sailing.