In the USA, the government cannot have copyright and everything is in the public domain. This is not the case in many other countries.
This is an overly broad statement. Federal government agencies in the USA can hold copyright in certain situations, and the rule doesn't apply to state or local government.
This is a really interesting page that lists ways that the government can own copyrighted works.
(Also the design that Apple lifted is trademarked, not copyright, but that's a different issue).