At the moment, it really seems like the only reason Apple is being fixated on is because they're the ones that are publishing some of the most detailed first-hand accountings of what's going on within their supply chain. Greater transparency, in this case, is translating to a disadvantage. I don't think they're the only ones, but some cursory Googling reveals plenty of companies with codes of conduct, and some even with detailed explanations, but few if any with detailed reports, annual or otherwise.
In essence, they're being proactive—in part because they can afford to be—and getting skewered because they're giving people a peek under the hood.
That's one part. The other is of course that they are wildly profitable, in part through large profit margins, and the question arises of “can't you cut into those margins to pay your workers better”. Though this latter one is not only true of Apple, Apple is perhaps one of the most visible companies to whom it can apply.