Apple allows you just as much control as every other product. If you were to break all of the security controls you could do whatever you want including adding a new App Store just like Cydia did.
The question is whether Apple should make it easy for you to do this. And there is no legal basis for that.
Sure there is. Section 2 of the Sherman Act makes it illegal to "monopolize, or attempt to monopolize, or combine or conspire with any other person or persons, to monopolize any part of the trade or commerce among the several States, or with foreign nations." You may think that Apple's conduct does not fall under that provision, and it remains to be seen whether the federal courts will agree, but if the legal argument were totally baseless then Apple wouldn't have gone to the Supreme Court to shut it down at a preliminary stage.
