The feds are not (usually) allowed to get away with owning copyrights (Section 105). The material contained on Interstate highway signs belongs to the public domain (lovely white letters reading "Wichita 36" on a green background are a part of America's heritage after all -- unless, of course, someone in Eisenhower's highway department licensed "Interstate green" from Pantone). But since states are allowed to hold copyright, it is conceivable that someone might fuss if I take a snapshot of their sign. If for example, I were to use my photo of the state of Pennsylvania's keystone shape as a prominent part of a highway billboard I've designed, then I might run into trouble with the state if they think I have falsely conveyed the state's support for my business, or if they otherwise are fastidious in protection of copyright. Likewise though, certain signs and marks of authority of the US government, while not protected by copyright, may be governed by separate laws -- e.g., the presidential seal, and US currency. ( Flags, stamps, seals, currency, all these things are rather complicated, it seems. The first sale doctrine, for example, does not seem to apply to flags.) Willie Nelson seems to be able to use the flag of the State of Texas without trouble, but this may stem from some inalienable rights of Texans. (Only speculation... I'm not fluent in Texan.)