Provided a friend of mine agrees to let me borrow (and copy) their media containing music / movie / book / whatever, what entitles you to interfere with such agreement?

Especially since that agreement didn't involve you.

There's no $deity-given right to control what happens to stuff you wrote / designed etc, once it's been published. Copyright is, sorry was, a legal construct meant to promote people creating artwork.

Once it overshot that intent bigtime, there's no justification for keeping it around. At least not in its current form.