Apple does a lot of things that probably don't have sound business reasons behind them in the name of control (which, in turn, they often claim is a business reason: I have yet to see any good evidence of this, however, only correlation that "Apple is successful" and "Apple is controlling", but no attempt to control for "Apple actually builds good things occasionally, and their competition is often incompetent in their attempts to build the same things, at least for the high-end market that scores you a lot of the $-points that people using attempt to score 'successful' with").

In this case, iOS is the one platform (well, technically Android could, and there is probably at least a couple handset manufacturers who decided to do this, but I can't come up with any off the top of my head) that requires you to pay Apple a subscription fee (the amount being unimportant: the key being that it allows them to better limit and control access, gives them options on how to track people or ban them) even if all you want to do is compile a normal application (not the fancy stuff we jailbreak for, but a regular-old boring "app") and load it on to you own device.

In contrast, Microsoft and Google only do this if you want to distribute your software using their marketplaces: you can, for free, "side-load" applications, with at most having to turn off some protection switch under Settings. (In the case of Microsoft, you still need an account, but as someone else pointed out it seems to be free; personally I think that actually is already out of scope for a GPL application, at least under a license like GPLv3 if not GPLv2; I am probably in the minority opinion on this: though, I'd find the FSF's opinion interesting.) Apple probably believes there is a business reason for this.