Again, I repeat, there is no law anywhere..domestic, international, US, Canada, Europe, that bans pay toilets on airplanes.
That said...you will never see it. The first time a passenger refused to pay and decided to "do his business" somewhere else on the plane would open up that airline to many lawsuits as well as possible health code violations.
Additionally, who's going to fly an airline that has them. Charge for food, fine if allowed to bring your own. Charge for bags as long as you can carryon for free.
Yet how many people can "hold it" for a ten hour flight across the pond.
So, while you will never see pay toilets on an airliner, there is no law banning them. And if there is, I wish someone would point it out to me. (And if there is, I'll admit I was wrong.)