While the state may be required to do so as a practical matter, it does not appear that it is required to do so as a legal matter. Even though all power of a subdivision devolves from a state, the subdivision may still contract independently. See here, for example:
It is the opinion of this office that the state is not required to assume liability in the event of default of bonded debt by a county. The county may contract in its own right and is liable on those contracts as an entity separate from the state.
While a county in its governmental capacity acts merely as an arm of the state for the convenient administration of government in its capacity for public good and on behalf of the state rather than for itself, it also acts in a proprietary or private capacity mainly for its own ends, purposes and benefits, separate from or in addition to the burdens and benefits imposed, or conferred, upon it by state government. Jones v. Haynes, 221 Tenn. 50, 424 S.W.2d 197 (1968). In this state, counties have the character of a public municipal corporation, with limited powers, and are liable as such. Their powers, duties, and liabilities are generally conferred by statute. Weakley County Municipal Electric System v. Vick, 43 Tenn. App. 524, 549–50, 309 S.W.2d 792 (1957); see also § 5–1–105, T.C.A. (Suits may be maintained against a county for any just claim as against other corporations). A county can, then, in its corporate capacity bind itself by contract, unless the contract violates the provisions which created the county. See West Tennessee Power and Light Co. v. City of Jackson, 97 F.2d 979 (6th Cir. 1938), cert. denied 305 U.S. 625 (1938) (contract by a municipal corporation). A county would have no more right to repudiate its contract than has a private person unless it deals with a governmental function. See Salt Lake City v. State, 448 P.2d 350, 354 (Utah 1968) (held: city, in selling water, was engaged in a proprietary activity).
Tenn. Op. Att’y Gen. No. 80-460 (Sept. 19, 1980)
Or, here:
Those cases stand for the proposition that the debts of an independent authority are not debts of the State under the Debt Limitation Clause. They rely on the legal autonomy of the issuing authority and on specific language disclaiming any enforceable obligation on the part of the State . To the extent that they rely also on the availability of revenue sources for debt payments, or even recognize that revenue sources are available, the cases directly or indirectly invoke the Special Fund Rule…
Lonegan v. State, 174 N.J. 435, 449-50, 809 A.2d 91, 99 (2002)
Huh? The federal government is the creation of the states. The states are not the creation of the federal government.
A state cannot obligate or encumber the government of the United States.