Other laws still exist too.
Owning pillows is legal. Killing people is not legal. Just because owning a pillow is legal doesn't mean you can smother someone to death with it.
I would hope that someone diagnosed with a mental disorder such as Alzheimers would lose their drivers license (not sure if this is the case), but if they were subsequently cured and passed a new driving test, then would you have a problem with that?