It's not that they can't, it's just pointless to offer a licence for something that's in the public domain. Also it's important to remember these works are not

It's not that they can't, it's just pointless to offer a licence for something that's in the public domain. Also it's important to remember these works are not in the public domain because someone declared them to be, they are simply because they are works carried out by the US government. Similar to how copyright is automatic, it's not applied only when you put the copyright symbol, that's just informational.