Article 1 Section 2 of the Constitution gives Congress the authority to levy taxes based upon apportionment among the states.
In 1909 the Constitution was amended to allow the income tax to not have to meet the enumeration requirement. The penalty for not having health insurance, under the Affordable Care Act, is now being described as a tax. But it's not based on enumeration or on income.
Also, it is being touted that it will only be collected from those without insurance who also have an income tax refund coming as there are no other enforcement options.
Regardless of this being a constitutional tax, which the Supreme Court seems to think it so, there are target tax filers who will bear the brunt of both not having health insurance and being due a refund: the working poor, the impoverished and the unemployed.