So John Roberts looked out and saw Antonin Scalia trying to steal his court, and he said, "Not so fast buddy." Or words to that effect. The Chief Justice is, of course, a fully paid up member of the Judicial Right. He subscribed to the ahistorical notion of free speech that spawned Citizens United. He was A-OK with Scalia's appeal to the Sovereignty Movement in his immigration opinion, but not so happy with the Jesuit's potshot at the president's recent executive action. What got under Roberts's skin was the likelihood that a decision overthrowing the ACA would be seen as a pre-emption of congressional power and a direct attack on the executive's right to govern. That would be fine for a Scalia Court, but no good at all for the Roberts Court.
In addition to which, it is entirely possible that John Roberts has a heart. Catholic yes, Jesuit no.