Unjailbroken iOS users, yes - certificates are signed by Apple. That said it's pretty easy to get a developer certificate for yourself. You just have to a) own a Mac and b) agree to not use it externally (which is what both Facebook and Google failed to do), but nothing prevents there being e.g. a community of people running open-source apps that don't abide by the App Store restrictions, all compiling them on their own machines.