At the time, many in the press didn't include the word "ultimately." I noted at the time that this indicated to me that he knew the decision of the court and was trying to undo it thru this pressure or perhaps blackmail. There are indications that Roberts changed his mind at the last minute.
This is truly frightening if the WH using whatever means can intimidate SCOTUS. Roberts had to manufacture a reason to uphold Obamacare. It was painfully obvious. The real question is why as you point out.
As Mark Levin points out, Roberts became a judicial activist with this decision. He created a precedent that the taxing power of the government can force people to buy something they don't want and it applies to every person in this country--except illegal aliens and some other excepted classes, which makes it difficult to defend as a tax.
Certainly food for thought
A feeling he does have towards DOMA which was overwhelmingly bipartisan.