> But, for example, Apple doesn't care in the slightest if I make a hardcore pornography app, sign it with my own developer cert, and install it on my phone.
You don't know that. They don't know you've done this and they can't know you've done it, so how you do you if they care or not?
You're acting like Apple has some vast conspiracy to eradicate objectionable material from the face of the planet. That's ridiculous.
Apple has been pretty open about the fact that they just care about what's on their storefront. The only reason that this effectively means they control what's on your phone is because most people can't install apps on their phone except via Apple's App Store. Although, as usual, everyone in the world is free to view whatever objectionable website they want.
You don't know that. They don't know you've done this and they can't know you've done it, so how you do you if they care or not?