I think people want to flip the cap away from them. The way the cap is supposed to open feels awkward when a right handed person tries to fill the tank from the side stand side of the bike. However, if you use a tank bag, the zippers lead you to flip the tank bag back onto the front of the seat. With it in that position, the way the tank opens makes sense.
It's no big deal to reverse the tank opening. Just cut a notch in the cap's flange so water can get to the drain hole. Heck, as long as you don't reverse the gasket under the filler cap, I am pretty sure the water will find it's way to the drain.