From Findlaw.com
Constitution: Sixteenth Amendment
INCOME TAX
History and Purpose of the Amendment
The ratification of this Amendment was the direct consequence of the Court's decision in 1895 in Pollock v. Farmers' Loan & Trust Co.,1 whereby the attempt of Congress the previous year to tax incomes uniformly throughout the United States2 was held by a divided court to be unconstitutional. A tax on incomes derived from property,3 the Court declared, was a ''direct tax'' which Congress under the terms of Article I, Sec. 2, and Sec. 9, could impose only by the rule of apportionment according to population, although scarcely fifteen years prior the Justices had unanimously sustained4 the collection of a similar tax during the Civil War,5 the only other occasion preceding the Sixteenth Amendment in which Congress had ventured to utilize this method of raising revenue.6
Excise taxes are taxes paid when purchases are made on a specific good, such as gasoline. Excise taxes are often included in the price of the product. There are also excise taxes on activities, such as on wagering or on highway usage by trucks. Excise Tax has several general excise tax programs. One of the major components of the excise program is motor fuel.
It sucks that they're taxing what every person needs to do to live, except for those who can successfully live under the table, but that doesn't mean it's necessarily unconstitutional or beyond the taxing power of Congress.