Recall that this is the case in which abortion clinics and abortion rights organizations sought an injunction against anti-abortion trespassers. The federal government filed a friend of the court brief arguing that the federal civil rights statute didn't apply to this behavior, and that the behavior was properly punished by state criminal law and tort law. (For more on the legal issue, read the Court's opinion.) John Roberts cosigned the brief, and delivered the government's oral argument.

I thought I'd add one detail, though, that people haven't noted: Not only was the government's position -- the one that Roberts is faulted for arguing -- accepted by six of the nine Justices, those six included two Justices who voted in favor of recognizing abortion rights in Casey v. Planned Parenthood. Justice Kennedy signed on to the majority opinion in its entirety. Justice Souter agreed with the majority on the issue that had been briefed by the government, but dissented in part because he thought another issue -- which the government's brief and oral argument, as best I can tell, had never addressed -- might be a winner for the plaintiffs. (The majority thought this separate issue hadn't even been presented below; Justice Souter argued that it had been adequately raised; but the government apparently didn't think it was in play, and therefore didn't address it.)

Whether this particular federal statute, as interpreted by the Supreme Court over many decades, should be read as barring illegal private interference with abortion rights, is a contested question -- the Court did split 6-3 on it (Justices Blackmun, Stevens, and O'Connor were in the dissent, Rehnquist, White, Scalia, Kennedy, Souter, and Thomas in the majority). But, as others have pointed out, one can certainly oppose criminal trespass and obstruction of entrances and yet think that this is a matter for state law, not federal law as written in the 1870s and interpreted by the Court since. And it's quite clear that this is an eminently mainstream position, taken by centrists and liberals Justices (e.g., Justices White and Souter) as well as conservatives, and by pro-abortion-rights people (e.g., Justices Kennedy and Souter) as well as by people who believe the Constitution does not secure abortion rights.