Insurance Mandate as a Tax?

I'm no constitutional scholar, but I have read the Constitution. It seems to me that our government is allowed to levy tariffs, and by the 15th amendment, an income tax. Now we have the administration claiming that the health insurance mandate is a tax, justified under the "power to lay and levy taxes." I don't see how this tax fits within the definition of a tariff or income tax. Maybe it's a tariff because you're importing all that fat and poor health.