For example, if a US citizen used part of the Constitution in an essay would that person not need to cite it?
Statutorily, any copyrightable work created by a federal employee as part of their job is in the public domain, so the constitution is in the public domain (not to mention that most of it was created long enough ago that a copyright would have expired).
But you're confusing plagiarism and copyright violation which are two different things. Plagiarism is representing someone else's work as your own. If I present, say, "I wandered lonely as a cloud" as my own work, that's plagiarism, even though the copyright on Wordsworth's poem is long expired. The penalties for plagiarism are generally extra-legal. In academic settings, it might mean getting a failing grade on the work in question or possibly the whole class, or in extreme cases, even expulsion from the school. In working settings, it might mean losing one's job or facing other sanctions from the employer such as being denied a promotion or facing a demotion or pay cut. In publishing, cases of plagiarism often result in the withdrawal of the published work and the return of any moneys paid for publication. (Although in most cases, the punishment may be symbolic or non-existent. I remember being in the room when two professors were discussing whether it was worth the trouble to deal with an obvious case of plagiarism in a student's paper.)
Copyright violation, on the other hand, is a legal matter, and refers to unauthorized use/duplication of the work. If I reproduce a work under copyright without permission (or a significant fraction of that work), it doesn't matter if I attribute it to the original creator or not, it's still a legal violation and I can be sued.
That said, other than the fact that the work of federal employees as part of their job being statutorily public domain, most of this is a legal or moral question rather than a political one. Also, given that I'm not a lawyer and I've not read in depth on copyright law in 30-some-odd years, I wouldn't take this post as a guide for any sort of legal questions.