The US Copyright office states that such works are not subject to copyright, so the legal aspect is clear.
That doesn't mean it shouldn't be attributed, though. By that argument, anything you copy that is not copyrighted does not require attribution (such as public domain works). Regardless of the technical legality of doing such a thing, that is not acceptable behavior on our network.
Think of it in the inverse. If you do not attribute that the content was not created by you, it is assumed that you are trying to claim copyright for it as your own work, which you did not create. How would anyone else know otherwise?
Attribution is about more than just protecting copyright. It emphasizes that pieces were created by someone else and not you, which can be important information in determining the validity of an answer, where it can be used elsewhere, or how to properly attribute another reproduction. If it wasn't created by you, attribution is always required here. We do not make exceptions because the content being attributed is not copyrighted.