The PTA isn't stopping at walled-garden App Stores; Google and Apple are just one of the few countries large enough to have a physical presence Pakistan can threaten. They also threatened a handful of US-based web hosts who are basically prohibited from seeing their families until they censor this sect.

(And yes, I'm using the word 'censor' here. It is entirely appropriate under even the narrowest definitions of the word, as the decision to remove content was made with the force of law. The US has similar provisions known as the 'state actors doctrine'.)