It comes down to an opinion by Jimmy Carter's second Attorney General, Benjamin Civiletti[1]. The gist of his opinion, which I need to find a better link for,

It comes down to an opinion by Jimmy Carter's second Attorney General, Benjamin Civiletti[1]. The gist of his opinion, which I need to find a better link for, is that since Congress has sole authority over the budget it is illegal for the government to spend money that was not preauthorized because it undermines Congress' authority. Furthermore, he cited some obscure statute as the reason volunteering to perform normally paid functions would be illegal. [1] http://www.npr.org/2011/04/08/135247483/who-raised-the-stakes-in-government-shutdown http://www.npr.org/2011/04/08/135247483/who-raised-the-stake... Edit: His opinion was based on a strict interpretation of the Antideficiency Act[2], the key provisions of which are: The Antideficiency Act prohibits federal employees from * making or authorizing an expenditure from, or creating or authorizing an obligation under, any appropriation or fund in excess of the amount available in the appropriation or fund unless authorized by law. 31 U.S.C. § 1341(a)(1)(A). * involving the government in any obligation to pay money before funds have been appropriated for that purpose, unless otherwise allowed by law. 31 U.S.C. § 1341(a)(1)(B). * accepting voluntary services for the United States, or employing personal services not authorized by law, except in cases of emergency involving the safety of human life or the protection of property. 31 U.S.C. § 1342. * making obligations or expenditures in excess of an apportionment or reapportionment, or in excess of the amount permitted by agency regulations. 31 U.S.C. § 1517(a). [2] http://www.gao.gov/legal/lawresources/antideficiencybackground.html http://www.gao.gov/legal/lawresources/antideficiencybackgrou...