"The theory, which once won a qualified approval, that a tax on income is legally or economically a tax on its source, is no longer tenable"
Graves v. New York ex rel. O'Keefe, 306 U. S. 466, 480 (1939)
What you have quoted from is an tax immunity case having nothing to do with whether or not the Congress may expressly exempt business or individuals from paying any Federal tax it chooses to enact within it express powers to levy, or repealing any provision of law it has enacted.
Graves v. New York ex rel. O'Keefe, 306 U. S. 466, 480 (1939)
- We are asked to decide whether the imposition by the State of New York of an income tax on the salary of an employee of the Home Owners' Loan Corporation places an unconstitutional burden upon the federal government.
- Congress has declared in 4 of the Act that the Home Owners' Loan Corporation is an instrumentality of the United States and that its bonds are exempt, as to principal and interest, from federal and state taxation, except surtaxes, estate, inheritance and gift taxes. The corporation itself, 'including its franchise, its capital, reserves and surplus, and its loans and income,' is likewise exempted from taxation; its real property is subject to tax to the same extent as other real property. But Congress has given no intimation of any purpose either to grant or withhold immunity from state taxation of the salary of the corporation's employees, and the Congressional intention is not to be gathered from the statute by implication.
- Whether its power to grant tax exemptions as an incident to the exercise of powers specifically granted by the Constitution can ever, in any circumstances, extend beyond the constitutional immunity of federal agencies which courts have implied, is a question which need not now be determined.
- Assuming, as we do, that the Home Owners' Loan Corporation is clothed with the same immunity from state taxation as the government itself, we cannot say that the present tax on the income of its employees lays any unconstitutional burden upon it. All the reasons for refusing to imply a constitutional prohibition of federal income taxation of salaries of state employees, stated at length in the Gerhardt case, are of equal force when immunity is claimed from state income tax on salaries paid by the national government or its agencies. In this respect we perceive no basis for a difference in result whether the taxed income be salary or some other form of compensation, or whether the taxpayer be an employee or an officer of either a state or the national government, or of its instrumentalities. In no case is there basis for the assumption that any such tangible or certain economic burden is imposed on the government concerned as would justify a court's declaring that the taxpayer is clothed with the implied constitutional tax immunity of the government by which he is employed.
Hmmm this case says nothing as regards what the Congress can chose to do as regards exempting a Federal contractor, or anyone else for that matter, from paying any kind of Federal tax it chooses to give exemption to.
The only thing that can be said about the case is that it reminds us that government employees (federal or state) do not have implied immunity from state or federal taxation as a matter of constitutional law.
Now looking at your excerpt in context:
- The present tax is a non-discriminatory tax on income applied to salaries at a specified rate. It is not in form or substance a tax upon the Home Owners' Loan Corporation or its property or income, nor is it paid by the corporation or the government from their funds. It is measured by income which becomes the property of the taxpayer when received as compensation for his services; and the tax laid upon the privilege of receiving it is paid from his private funds and not from the funds of the government, either directly or indirectly. The theory, which once won a qualified approval, that a tax on income is legally or economically a tax on its source, is no longer tenable, New York ex rel. Cohn v. Graves, 300 U.S. 308, 313 , 314 S., 57 S.Ct. 466, 467, 108 A.L.R. 721; Hale v. State Board, 302 U.S. 95, 108 , 58 S.Ct. 102, 106; Helver- [306 U.S. 466, 481] ing v. Gerhardt, supra; cf. Metcalf & Eddy v. Mitchell, 269 U.S. 514 , 46 S. Ct. 172; Fox Film Corp. v. Doyal, 286 U.S. 123 , 52 S.Ct. 546; James v. Dravo Contracting Co., supra, page 149, 58 S.Ct. page 216; Helvering v. Mountain Producers Corp., 303 U.S. 376 , 58 S.Ct. 623, and the only possible basis for implying a constitutional immunity from state income tax of the salary of an employee of the national government or of a governmental agency is that the economic burden of the tax is in some way passed on so as to impose a burden on the national government tantamount to an interference by one government with the other in the performance of its functions.
Express exemption by Congress is not "constitutional immunity." and Congess can exempt the payment of any tax it has authority over, just as it is not required to levy any specific tax at all. The power to do a thing is not a demand to exercise it only the authority to do so as Congress will.