The technical barrier. Maybe I'm ignorant, but I think Chrome & Firefox would put up a much bigger fight than App/Play Store. The latter is already a walled garden.

The US doesnt seem to have a problem taking down websites its found to be illegal, I don't know why a PWA would be much different. They dont need any buy-in from browser developers.

Pretty sure they do this by taking over the domain... Not sure if that would work in this case...