The Constitution protects the sovereign citizens from taxation in two ways: direct taxes must be apportioned (i.e. directed to the states rather than the citizens), and the other classes of taxes must be uniform (i.e. levied against actions, which are usually avoidable). The income tax has been ruled avoidable because you can avoid income up to a point; yes, that's the short version. If we went to consumption tax, you're right it would amount to the same even if they change the category, because they would say consumption is avoidable. Paging ConsumeProduct.