Distribution restrictions on top of GPL and they don't own the copyright on all the code without a CLA which prevents licensing under something other than GPL for the App Store.

iPad, iPod, iPhone, MBP, iMac, AppleWatch, ... oh, that's already six devices tied to the same account.

I think the idea is that you don't tie all your friends' devices to your account to share apps etc., but I am just guessing.