Almost everything I've read about Joan Biskupic's presentation of NFIB v. Sibelius asserts that it confirms the already established story that Chief Justice Roberts "switched" his vote in the case. Concluding her chapter, Biskupic does refer to Roberts's "change of heart." The account she actually offers, though, does not show a switch or a change of heart. (I take some satisfaction that her account mostly confirms the inferences I drew in In the Balance -- and that the divergences are mostly on matters that I speculated about but that she doesn't discuss.)

Here's what Biskupic says:  At the conference discussion on the case, Roberts "felt just as strongly about the commerce-clause power as did the other four in the majority." He "voted to uphold the Medicaid expansion, yet he expressed some tentativeness on this provision." "No vote had been taken related to congressional taxing power."

Then Roberts began to draft his opinion. His draft maintained the position he had taken at the conference, that the individual mandate could not be sustained under the commerce clause. He apparently thought that this would invalidate only two provisions, community rating and coverage of preexisting conditions. [Justice Kennedy (correctly, I think) rejected that position, taking the view that without those provisions the entire system would be unsustainable (and so should fall because the two provisions were inseverable from the entire system). But this, it seems to me, is peripheral to the "changed his mind" story.]

After working on the commerce clause issue, "in mid-April, Roberts moved into new territory" -- the possibility of construing the individual mandate as a tax and upholding the mandate under that clause.
"Roberts decided that he could make a case for the taxing-power grounds." "By early May," he was drafting an opinion doing just that.

This account DOESN'T SHOW THAT ROBERTS CHANGED HIS MIND on anything. He thought from the beginning that the mandate was unconstitutional under the commerce clause, and continued to think that. And he hadn't thought anything at all about the tax issue, but when he turned his mind to it, he concluded that the mandate could be construed as a tax and as such was constitutional.

What about Medicaid expansion? Remember, he "expressed some tentativeness" about its constitutionality. Biskupic doesn't say much about Roberts's thinking about this issue as he worked on his opinion, but it's seems reasonably clear that his view about "coercion" was the way he converted his initial tentativeness into a holding.

Biskupic writes, "Breyer and Kagan disagreed with Roberts's new thinking on Medicaid, but they were pragmatists." "They were willing to meet him halfway" "if there was a chance that Roberts would cast the critical vote to uphold" the individual mandate. And, she says, "negotiations in May were such that they still considered that a shaky proposition." The implication here is that Breyer and Kagan voted with Roberts on limiting the Medicaid expansion to solidify his vote on the tax issue. The timeline here is fuzzy, and -- in my view -- without additional information (such as identification of the sources she had and the precise statements they made),* the account seems to me not entirely plausible or consistent with the dynamics of opinion-drafting. [To put it as clearly as possible, it strikes me as highly unlikely that anyone said or thought, "I/he (John Roberts) will drop the section on the tax issue unless they/we (Breyer and Kagan) go along with me/him on Medicaid expansion."] And maybe it's worth pointing out that, on this account, the only people who "changed their minds" were Breyer and Kagan.

The most parsimonious explanation is consistent with the account Biskupic provides: The justices -- all of them -- voted as they did because that's what they thought the best view of the law required. (Just to be clear, that doesn't mean that I think they were just calling balls and strikes. Their best view of what the law requires is political all the way down -- but it is also, as Justice Kagan said, law all the way down too.)

* I personally would be skeptical were the material to be sourced to law clerks, who generally think they understand the Court's dynamics better than they actually do.