IANAL, but it is my understanding that at issue here is the deal would (could) have created / extended a monopoly. Antitrust in the US does not disallow monopolies, per se, as you note; however, it does disallow creation or furthering of a monopoly via contracts, M&A, conspiracies, collusion, etc.
From US Code Title 15, Chapter 1, § 1:
Every contract, combination in the form of trust or otherwise, or conspiracy, in restraint of trade or commerce among the several States, or with foreign nations, is declared to be illegal.
Source: http://www.law.cornell.edu/uscode/html/uscode15/usc_sec_15_0...
The other sections of that chapter discuss some specifics (such as §18, which specifically covers M&A) and there is a wealth of case law backing it up and clarifying, as well.