Earlier quoted context omitted.
"Competition law, known in the United States as antitrust law, is law that promotes or maintains market competition by regulating anti-competitive conduct by companies."[1] One could argue that rejecting apps that make use of (or in this case encourage signup to) Dropbox is anti-competitive conduct since Dropbox is a competitor of iCloud (isn't it?) [1] http://en.wikipedia.org/wiki/Competition_law
It is hard to name any proprietary product platform, from cars to KitchenAid mixers, which would not run afoul of this common nerdly definition of "antitrust". In reality, the presence of very viable substitutes in the market for the iPhone (to wit: Android) probably moots the argument here. To go further with this argument, you will need to venture beyond Wikipedia summaries.
> In reality, the presence of very viable substitutes in the market for the iPhone (to wit: Android) probably moots the argument here.