In a bit of a surprise, Carr won on a 6-3 vote. Although, as I thought, the likely way Carr would be victorious would be on the statutory interpretation issue and not the Ex Post Facto Clause claim. The outcome will have an effect on very few prosecutions and Congress can "correct" the outcome whenever it wants. So, it isn't much of a decision against SORNA or the AWA in any meaningful way.
I have to say that I was more than a little disappointed that Justice Alito, who had to address the Ex Post Facto issue in his dissent, disposed of it in a single footnote paragraph which lacked any substance. Here was his argument on the issue:
Petitioner makes the additional argument that interpreting §2250(a)(2)(B) to reach pre-enactment travel renders the statute anunlawful ex post facto law. See U. S. Const., Art. I, §9, cl. 3. Petitioner remained unregistered in Indiana five months after the promulgation of the regulation making SORNA applicable to persons with pre-SORNA sex-offense convictions. For essentially the reasons explained by the Court of Appeals, see United States v. Dixon, 551 F. 3d, at 585– 587, I would reject petitioner’s ex post facto argument.
Even though it isn't a majority opinion, that treatment is just sad. Judge Posner's lower court opinion of the Ex Post Facto Clause claim was based upon lower court cases that were inapplicable (because they applied in instances where Congress specifically declared that the crime was a "continuining offense"). To just adopt such a poorly developed argument without engaging any of the briefing on the issue seems weak.
Of course, everyone has noted the odd alignment of the Justices as Thomas, Alito, and Ginsburg formed the dissenting group. I think if more were at stake in the case, such a grouping would have been unlikely.