If I understand correctly, antitrust concerns would only kick in if Apple uses its monopoly power to screw its competitors. For example, if iOS apps were required to sell content for less at the App Store than through competing channels, then Amazon et al. could rightfully cry foul.

But screwing your customers is not an antitrust violation: it’s capitalism as usual. :-)

Given the impact of these new policies on music streaming services like Rhapsody et al and Apple's dominant share in the digital music and digital music player markets, I think antitrust will become part of the story sooner rather than later. Especially if Apple launches their own streaming service (as has been rumored).

Antitrust is about barriers to entry - the market is still wide open for anyone to compete with Apple.