In my opinion, the Obama administration had some embarrassing info on Roberts and threatened to publish it. According to other judges on the bench, Roberts started out as against the ACA.
Although I'm not a lawyer, I work in the legal business. I understand (somewhat) Roberts' decision. People who don't work in the law often have trouble understanding legalities vs. feelings or the ultimate effect of a ruling, or even that a judge may not like having to make a particular ruling, but the law requires it.
Roberts decided that, based on other laws and statutes, the so-called penalty is, in fact, a tax. He may have hated having to rule that way. But if that's how he saw it, legally speaking, he had no choice but to rule that way. If he had decided otherwise, there might be other so-called penalties that would have fallen because they wouldn't be considered taxes, either.
Roberts didn't betray anyone. He came to a decision based on the law, as he saw it, as the law was presented to him by both sides. And whether you agree with that particular decision or not, it is a GOOD thing that a person comes to a legal ruling based on the law rather than how he FEELS about it and despite what his preference would be for the outcome.
Sotomayor has decided a couple of cases that were disappointments to Democrats. But, like Roberts, it's a good sign when a person rules a way that he or she probably would prefer the law let him/her rule another way.
BTW, justices have on occasion mentioned this fact, that they have to rule a certain way because of a statute or regulation, and that the fix for that would be the changing of that statute/regulation. That is the answer to a legal court opinion that results in an outcome that most people don't like. The answer it to cure it by congressional statute.
Congress could have fixed this, you know, but chose not to. Instead, the House chose to vote to repeal the ACA over 40 times, knowing it was a waste of time. So I ask you....how serious were the Republicans about fixing the ACA for the people. Or has it just been political all along?