I'm not sure if there are any specific laws against it, though you'd really think that there probably should be, at least as the illness progresses. However in most cases it is suggested that family members talk with their loved one early on, after diagnosis, about the fact that they will at some point no longer be able to drive themselves.
It's a tough situation to be in, and I think that as long as one's condition is being properly managed, then they are probably ok. It's when things start deteriorating that is the major concern.
My father in law, got sick with that and nearly killed a kid on his bike, so hide all the keys drain the fuel, and tell them it;s broke, thats what I did then 6 weeks later he passed away, the kid is ok, too, you really need to keep them in an environment that is safe.