> You can't copyright a national flag,

Except maybe the US flag in the US (given the particular US prohibition on copyright of federal government works), I don't see why not; and, actually, if the US were to adopt a private design as the official national flag rather than commission it, I'm not sure it would be a problem even in the US, since other private works adopted as national standards but not created by government in the US remain protected by copyright.

There are certainly ways a private copyright on a national flag is inconvenient, but inconvenient and impossible aren't the same thing.