His reasoning is something that I've seen elsewhere today. Roberts along with the majority opinion holders have put a limit on what can be done under the commerce clause, something that past courts have not done.
By calling this a tax, and a huge one at that, it gives the republicans more ammunition for repealing the entire bill. However, some insurers have already said that they will keep the "children up to age 26" provisions regardless. (It's a win-win for them anyway.) They'll keep provisions that benefit them even if ObamaCare is repealed by the incoming majorities of republicans.
