The real tricky part is the fine print. Apple doesn't claim that the iPhone is excluded from their gift cards, and in fact says that more or less you can purchase whatever you want with them. The speculation is that they are treading into dangerous legal territory now. While people are already willing to sue over just about anything these days, it seems bizarre that Apple would make themselves such an obvious target in this case.
All of this comes back to Apple not wanting to people to mod or hack the iPhone – or at the very least not profit from doing such. I can understand the latter, but after so many years of Apple products being tweaked with it seems that they are losing touch with a large portion of their customer base. In fact, about a fifth of their iPhone customer base.