Playing Kremlinology here, it feels like Breyer originally had an opinion that explained that APIs were not copyrightable, but sacrificed that section of the opinion to build a 6-2 majority. It's really weird that the opinions took so long to come out for how simple they end up being--why is this coming it in early April instead of early/mid February if it's written like this? My supposition is that there was a much more sharply divided court, running a 3-3-2 or 4-2-2 opinion, and by dropping the discussion of whether or not the APIs were copyrightable and instead saying "it's at best thin copyright" (i.e., it's copyrightable but good luck ever winning infringement) the opinions collapsed down into a more simple outcome. One thing's for sure: this is the case I'm most interested in finding out all the backroom discussions that went on here.

It's a lot easier to say what Google did was fair use than decide where to draw the line on where an API is in a software project and whether it is copyrightable or not.