One of the puzzles of Jones is how Scalia’s opinion ended up being the majority opinion of the Court, while Justice Alito’s view is merely a concurring opinion. The puzzle is that the apparent 5th vote for the Jones majority, from Justice Sotomayor, wrote a concurrence strongly hinting that she would accept a far broader rationale something akin to that in Justice Alito’s concurrence in the judgment. The question is, why sign on to Scalia’s opinion instead of Alito’s?
There are a bunch of possible reasons, of course, but one possibility involves the timing of circulated drafts. The Chief assigned the majority opinion to Scalia, who had floated his theory of the case at oral argument. Imagine Scalia circulated his majority opinion quickly, and Sotomayor joined it pretty soon after that. Some time passed, and then Justice Alito sent around his concurring opinion. Justice Alito’s opinion is mostly a criticism of Scalia’s approach, but it then has a relatively brief pro-privacy section at the end that addresses questions not reached by Scalia’s opinion. Imagine Sotomayor read Alito’s opinion and really liked that part of Alito’s opinion. But she had already signed on to Scalia’s draft majority, and it’s considered bad form to un-join an opinion after signing on. It’s especially bad form if you followed the common practice of asking for a few changes to the draft majority opinion as a condition of signing it. Also, while Alito hinted at how he would decide the case, that section is relative brief and quite vague. So Sotomayor might have stuck with Scalia’s opinion as a matter of propriety and good internal court relations, and then written her solo concurring opinion indicating her agreement with much (although by no means all) of Alito’s opinion.
Of course, that’s just one possibility among many.
UPDATE: Over at SCOTUSblog, Tom Goldstein notes a point that I simply missed on my initial reading of the opinions: Alito’s concurring opinion not only rejects the new trespass theory, but further indicates that the installation and short-term monitoring is fine — it’s only long-term monitoring that Alito would say is regulated by the Fourth Amendment. So Sotomayor’s choice wasn’t between a narrow and broad theory, as I had initially surmised, but between two very different theories. Sotomayor joined one and indicated strongly that she would likely favor the other, but she didn’t need to reach that; doing would have required a United States v. Booker-esque combination of two sets of Justices, which in addition to being complicated wasn’t needed because at least the result was settled in this case.