I think he was influenced by the Lefts language during the decision making process. It was pretty much "if you strike down this law then we will ruin the image of the court", Roberts got cold feet as evidenced by his strange, poorly thought out and incomprehensible wording of his decision.
He had a chance to do right, but failed as he was influenced by outside forces which isn't supposed to happen.
This whole shit show sets a really, really ,really bad precedent.
Roberts originally voted to vote down the mandate and changed his vote at the last minute due to the assault by Leahy, Obama, the MSM, etc.
Notice how they focused on Roberts and not Kennedy beforehand to the vote? That was because it was leaked how Kennedy was so adamant that the entire law be struck down and Roberts was a little wishy washy.
Obama's thuggery and threats worked, sadly. Roberts, instead of upholding the reputation of the court, made a mockery of it by issuing a ridiculous ruling that that the mandate WAS NOT A TAX for the purposes of the Anti-Injunction Act, but could possibly vbe viewed AS A TAX for purposes of the rest. He ruled it against the commerce clause, but instead vastly increased Govt powers by issuing this insane ruling.
Of course to the "ends justify the means" Democrats, they could care less, but most people see this treason for what it is - bowing down to the Tyrant in Chiefs' threats.