This is from the Department of Justice. The web link is at the end. The 2nd note seems to say states have legal authority to use SS numbers for tax administration, so I’m guessing IRS does too.
OVERVIEW OF THE PRIVACY ACT OF 1974, 2004 EDITION
SOCIAL SECURITY NUMBER USAGE
Section 7 of the Privacy Act (found at 5 U.S.C. § 552a note (Disclosure of Social Security Number)) provides that:
“It shall be unlawful for any Federal, State or local government agency to deny to any individual any right, benefit, or privilege provided by law because of such individual’s refusal to disclose his social security account number.” Sec. 7(a)(1).
Comment:
Note that although this provision applies beyond federal agencies, it does not apply to: (1) any disclosure which is required by federal statute; or (2) any disclosure of a social security number to any federal, state, or local agency maintaining a system of records in existence and operating before January 1, 1975, if such disclosure was required under statute or regulation adopted prior to such date to verify the identity of an individual. See Sec. 7(a)(2)(A)-(B).
Note also that the Tax Reform Act of 1976, 42 U.S.C. § 405(c)(2)(C)(i), (iv) (2000), expressly exempts state agencies from this restriction to the extent that social security numbers are used “in the administration of any tax, general public assistance, driver’s license, or motor vehicle registration law within its jurisdiction.” See, e.g., Stoianoff v. Comm’r of the Dep’t of Motor Vehicles, 12 Fed. Appx. 33, 35 (2d Cir. 2001) (finding that plaintiff’s Privacy Act claim would fail because § 405(c)(2)(C)(i) “expressly authorizes states to require the disclosure of social security numbers in the administration of driver’s license programs” and further provides that “any federal law that conflicts with this section is ‘null, void, and of no effect’”); Claugus v. Roosevelt Island Hous. Mgmt. Corp., No. 96CIV8155, 1999 WL 258275, at *4 (S.D.N.Y. Apr. 29, 1999) (considering housing management corporation to be state actor for Privacy Act purposes but finding that Privacy Act does not apply to income verification process for public housing program because of exception created by 42 U.S.C. § 405(c)(2)(C)(i)). Exemption from the social security number provisions of the Privacy Act is also provided for certain other government uses. See, e.g., 42 U.S.C. § 405(c)(2)(C)(ii) (authorizing state use of social security numbers in issuance of birth certificates and for purposes of enforcement of child support orders); 42 U.S.C. § 405(c)(2)(C)(iii) (authorizing use of social security numbers by Secretary of Agriculture in administration of Food Stamp Act of 1977 and by Federal Crop Insurance Corporation in administration of Federal Crop Insurance Act).
“Any Federal, State or local government agency which requests an individual to disclose his social security account number shall inform that individual whether that disclosure is mandatory or voluntary, by what statutory or other authority such number is solicited, and what uses will be made of it.” Sec. 7(b).
http://www.usdoj.gov/04foia/1974ssnu.htm