Here's a comment from a lawyer reacting to Andrew Sullivan's live blog:

What I think a lot of people are missing in this case is its connection to that foundational Supreme Court case, Marbury v. Madison (to which the Chief Justice cites several times). Marbury is an interesting study; essentially it was a political battle between Jefferson and the Federalists. John Marshall, presiding Chief Justice, most definitely held with the latter. In deciding the case, Marshall pulled an impressive feat of judicial jujitsu; he found for Jefferson's side, but announced the principal of Judicial Review, something with which Jefferson would have disagreed. But Jefferson could hardly caterwaul about a case he had 'won.' Jefferson won in the short term, Marshall, arguably, in the long term. Thence to Roberts' opinion.

He did uphold the mandate as a tax, but specifically said it was beyond the Commerce Clause power. Much of the opinion is dedicated to explicating why this is so (and adopting the anti-ACA arguments for such). I think Roberts has taken a lesson from his famed predecessor. By upholding the mandate as a tax, but deciding the Commerce Clause issue against what would seem to be the weight of precedent, he may be hoping to hand conservatives the long-term win.

If Wickard v. Fillburn (generally seen as the broadest Commerce Clause case) is one day overturned, as seems likely, much of the language will be taken from the Chief Justice's opinion.

To my mind, Roberts didn't want to bless an expansion of the government's power, but even more didn't want to take new business away from the corporations that make up the health industry.

This is good on understanding what the ruling really said and what Roberts did (it contradicts your headline, BTW--the ruling did not approve a "mandate"):

'Looks Like a Tax': A Simple Guide to Today's Historic Supreme Court Ruling
By Derek Thompson, The Atlantic, June 28

[....]

And so, this is the way health care reform survives. Not as a mandate. But as a tax.

[....]

In the end, four justices voted to strike down the entire health care law. Four justices deemed it appropriate under the commerce clause. And, in the middle, Chief Justice John Roberts concluded that the entire health care law could live because the mandate in question can be legally justified under Congress's power to tax.

Here is his argument, simple as I can make it.

The health care mandate forces individuals to purchase insurance. This puts it in the vicinity of the commerce clause, which regulates interstate activity. But not buying health care, Roberts wrote, is more like interstate inactivity. And Congress doesn't have the right to regulate us while we're not doing something [....]

Thompson points out an interesting incongruity that may end up being more important than he thinks: that Obama has been known to argue that it's a mandate, not a tax!:

And that's exactly the conclusion that Roberts reached: Paying a penalty, which is calculated based on your revenue, to the IRS, in a way that raises revenue, is essentially like paying a tax. The individual mandate makes going without insurance "just another thing the Government taxes," he wrote, "like buying gasoline or earning income." And if going without insurance is just another thing the government taxes, it is protected by Congress's constitutional power to tax -- even if the administration doesn't want to call it a tax. Here's Roberts with the money graph:

Edit to add: And you know what? I totally buy Roberts' argument. We are free to live in sin rather than getting married, but we pay a tax penalty for doing so; we are free to rent rather than buy our home but we pay a tax penalty for doing so, etc. Sooo, here's the problem I wonder about: what's to prevent a GOP Congress removing the penalty? Or even a split Congress like we have now, in run up to elections, removing the penalty because a media campaign has made it an unpopular point of contention surrounding the election? In effect, getting rid of the "mandate"?

The sheer intensity of winger and GOP distress over this decision may, as one effect, prompt some progressive-minded folks to entertain the thought that maybe the legislation wasn't all bad after all.

"The Framers gave Congress the power to regulate com­merce, not to compel it, and for over 200 years both our decisions and Congress’s actions have reflected this un­derstanding. There is no reason to depart from that un­derstanding now.....

The individual mandate forces individuals into commerce precisely because they elected to refrain from commercial activity. Such a law cannot be sustained under a clause authorizing Congress to “regulate Commerce.”.....

Sustaining the mandate as a tax depends only on whether Congress has properly exercised its taxing power to encourage purchas­ing health insurance,.......... 

not whether it can.

 Upholding the individual mandate under the Taxing Clause thus does not recognize any new federal power. It determines that Congress has used an existing one."

http://www.latimes.com/news/politics/la-pn-excerpts-from-chief-justice-robertss-opinion-on-healthcare-reform-20120628,0,5775958.story?page=2&track=rss

Well reasoned. 

MEDICARE FOR ALL