The U.S. Constitution gives Congress the power to tax and spend only for the "general welfare." The Supreme Court has essentially left it up to Congress to determine what constitutes the general welfare. ["The line must still be drawn between one welfare and another, between particular and general . . . . The discretion, however, is not confided to the courts. The discretion belongs to Congress."] Congress doesn't take this responsibility very seriously, if its members even recognize it exists, as evidenced by Senate Majority Leader Harry Reid's comment: "The Founding Fathers would be cringing to hear people talking about eliminating earmarks."
Someone's Never Heard of the Constitution's "General Welfare" Clause?: