In a 5-4 ruling, the high court decided the individual mandate requiring people to have health insurance is valid as a tax, even though it is impermissible under the Constitution's commerce clause.
"In this case, however, it is reasonable to construe what Congress has done as increasing taxes on those who have a certain amount of income, but choose to go without health insurance," Chief Justice John Roberts wrote in the majority opinion. "Such legislation is within Congress's power to tax."
In short, a tax can be imposed on a negative activity, ie something you don't do. <<