Hmm. At first blush, it does seem like Apple could get themselves in hot water over this. Generally speaking, under the antitrust laws that I am familiar with, it is illegal to abuse your dominant market position; for instance, in the 90s, Microsoft could have gotten in trouble if they told a small component maker that they couldn't sell their components to Apple, lest Microsoft remove support from said component in the next version of Windows.
However, since Apple employs some of the savviest lawyers around, I suspect there is more to this than meets the eye. For one thing, with Apple's global market share (traditional PC only, not tablets), Apple does not enjoy a monopolistic position (if I recall, usually 70% share). Second, Asus is not a small company being pushed around.
Thirdly, and this is what I suspect lest Apple be imitating MotoMo, is that Pegatron is using the same assembly line that Apple had them build (including the very at-the-time unique uni-body aluminum assembly line), and that Apple possibly invested in this line and has some legal recourse about demanding exclusivity. We don't know the details of Apple's contract with Pegatron. But Apple throws a lot of money at suppliers, and likely has specific terms; this may fall under contract-law.
If there are no such terms, and as long as Apple does not meet the criteria of having a monopolistic position, I believe Apple is free to be cut-throat about this. Business is cut-throat. It's only when a company gets too much power and competition is stifled that the Feds will step in.