If a text frame is created on a master page, you can edit the content outside the master page - but the edits only override the one instance of the text frame.
And of course you can edit the contents of MUSIC frames on the master page outside of the master page - in fact you never do anything else!
I don’t think the issue is “editing the contents of a graphics frame on the master page outside the master page,” but the fact that the edits aren’t local.
Whatever, Daniel has explained why we got to where we are.