They programmed it like this so you wouldn't get 'stuck'.
If Fainted Pokemon could not use Strength, for example, you may get stuck in an area since you could not move the boulder if the only Pokemon in the party that knew it got knocked out. It's for the same reason that you cannot forget HM moves; so you cannot be forced to black out. Again, pure Game Freak logic. You'll have to get used to it, they're like this for everything.