The Justice Department is arguing that the penalty for failing to abide by the individual mandate is constitutional because it is a tax, and is therefore covered under the General Welfare Clause.  This is a red herring argument.  A penalty may indeed be a tax, but the penalty and the mandate are different things!

If Congress were to pass a law requiring every adult male to undergo a vasectomy after having two children, and imposed a penalty for non-compliance, we could use this same logic to call this penalty a tax.  Which of course has nothing whatsoever to do with the fact that the original requirement is clearly unconstitutional!  It is the vasectomy (mandate), not the penalty, that lies outside the enumerated powers.  And if the mandate is unconstitutional, there can be no penalty