Great quote from the dissent:
For all these reasons, to say that the Individual Mandate merely imposes a tax is not to interpret the statute but to rewrite it. Judicial tax-writing is particularly troubling. Taxes have never been popular, see, e.g., Stamp Act of 1765, and in part for that reason, the Constitution requires tax increases to originate in the House of Representatives. See Art. I, §7, cl. 1. That is to say, they must originate in the legislative body most accountable to the people, where legislators must weigh the need for the tax against the terrible price they might pay at their next election, which is never more than two years off.
In short, to uphold this law, Chief Justice Roberts had to join the four liberal justices rewriting the law, making a liar out of President Obama, Nancy Pelosi, and Harry Reid, all of whom told the nation that this was not a tax.