This is a very common misunderstanding of fair use. Fair use
is copyright infringement. It's just a legal defence when you are being sued. This is one of the big problems with fair use in the US. If you intend to use a fair use defence, you will almost certainly go to court. You will not get a summary judgement of "Oh that's obviously fair use, so we don't have to have a trial". This means you will have to pay a lot of money for a lawyer.
Having said that, IANAL and I could easily be wrong. This is not legal advice.
The real question (which seems already to have been answered) is are APIs a copywritable work, and if so is an implementation that follows the API a derived work? So is it an infringement if I simply implement the API? The original court decision said it is not an infringement, the appeal said it is and the supreme court decided not to hear a further appeal. That's why we are left with a fair use defence.
This is quite spectacularly bad for the industry IMHO, but there is not much we can do about it now other than to have US citizens lobby congress to pass a new copyright law (which seems pretty unlikely). If it turns out to be fair use, that will make it slightly less bad. However, expect rent-seekers to start coming out of the woodwork to sue for implementations of specifications anyway. They know that they can simply sue you for less money than it will cost to defend yourself with a fair use defence. Expect this to be the new patent troll.