Romney is not insisting it is a tax, he is saying and I quote: "Well, the Supreme Court has the final word. And their final word is that Obamacare is a tax. So it's a tax. They decided it was constitutional. So it is a tax and it's constitutional. That's the final word. That's what it is."
http://www.cbsnews.com/8301-18563_16...date-position/
He is just noting that it is the Supreme Court's final word, to say it is not a tax goes against the very Supreme Court ruling that upheld it.