If you are right by the law, what can they really do besides putting fear in you?
Sue you. Even if you win, you're still out the money and time it cost to defend yourself. If you're lucky, you might get awarded your reasonable costs, but that doesn't pay for the stress and time.
And if they have a de-facto monopoly situation, other rules come into play.
Point me to a competing service that delivers software for the iPhone, and i'll grant that it's not a monopoly.
From a regulatory standpoint, monopolies don't apply to markets consisting of individual products from individual companies, only an individual company dominating an entire market segment.
The are many smart phones out there, and the iPhone doesn't even have the largest fraction of that market -according to NPD, RIM is by far the dominant player in the US, with three of the top five best-selling smart phone devices.
...without any copyright laws, would there be any need for GPL?
Without copyright law, there would be nothing to enforce access to source code, therefore a company could take an otherwise "open" pool of code, modify it and distribute it without disclosing the modifications. Add a dash of DRM to defeat unauthorized (but otherwise legal, without copyright) redistribution of binaries, and you've got a closed system.
When bad men combine, the good must associate; else they will fall one by one, an unpitied sacrifice in a contemptible struggle. - Edmund Burke