I think there is a challenge in applying current anti-trust laws to a company like Amazon because Amazon's reach is more wide than it is deep. AT&T owned almost the entire telecom network in the United States, Amazon doesn't have anywhere near that kind of influence in any of it's businesses. What Amazon does have is a significant presence in many different (competitive) businesses. I can't think of an example of antitrust being applied in a similar situation in the past but I could be missing something obvious.
The former may be true; not sure about the latter. Existing anti-trust law could be sufficient to deal with them, if the powers that be had the will to use it.