Not sure what you mean by "very hard to easy", but AT&T isn't a monopoly in the legal sense. They're the only ones who officially support iPhone, but that doesn't mean that they are committing anticompetitive acts. Honda dealers are pretty much the only businesses allowed to sell brand new Honda Civics, but are they being sued by the Department of Justice for "acting as a Monopoly"? Nope.
But OK, let's play along with your word game here. If AT&T is a "monopoly" because Apple is working with them with iPhone, then Verizon is also a "monopoly" for not working with Apple. They are bigger than AT&T, and they are selling Android phones, which may over time develop a larger overall market share than iPhone. Verizon is using their market share to suppress sales of iPhone by not selling it themselves. Make sense to you?
Here:
http://en.wikipedia.org/wiki/Sherman_Antitrust_Act