Although I applaud the effect of this decision, I can't help but agree with Clarence Thomas that the reasoning behind it is not sound. It waffles back and forth between arguments for fair use and arguments that APIs are not (or at least should not be) copyrightable in the first place. You can't have it both ways. If APIs are copyrightable (and a plain reading of the text of the law appears to indicate that they are) then Google's copying was clearly not fair use. The purpose was clearly commercial, not educational or a parody. The API was not merely a fact or an idea, it was a specific embodiment that required effort to produce. And the copying clearly had a negative impact on the value of the original.
What really happened here is that the Supreme Court did an end-run around the law to paper over a major fuckup by Congress. On the one hand, I'm glad that they fixed the problem. But the way that they did it fills me with dread for the future because it undermines the rule of law.
For the record, I absolutely despise Clarence Thomas and everything that he stands for. But in this case I think he has a valid point.