if you look at the holes on the rim where the lug nuts come in contact with the rim one side is beveled to insure the wheel is centered on the hub. If you flip the wheel that bevel will now be a gap on the back side of the rim, and the contact area for the lug nug will be significantly reduced making it unsafe to operate.
Hope this helps.
Edit: when I went outside to look at my stock wheels, they are beveled on both sides. I dont' see any reason why you couldn't flip them, but I don't know why you would want to.