Duke's arsenal isn't all that impressive, with the requisite shotgun, pistols and rocket launchers. Or maybe not. Less funny are the occasions when Duke runs into a room and starts killing female strippers and prostitutes. Besides, the whole point of the game is to kill everything in sight. If there were a bunch of innocent humans running around the earth-bound locations, too, it would make a helluva lot more sense.
Nothing sells like controversy.
It's an extremely cynical marketing ploy, and it works. Gameplay is ultimately what we're interested in, and in this area, Duke is the king. If 3D Realms really wants to impress us, they can move beyond this type of gameplay. Perhaps that's part of their appeal - their familiarity. Kind of like the buddy flick and the romance novel, nothing really changes, but that doesn't stop people from spending their money on them.