>That does not - of course - give WC the right to steal 25% of the value of MJ's work just because YT says they can.
This is completely wrong. Youtube can do whatever it wants unless MumboJumbo specifically signed a contract with Youtube to receive guaranteed compensation. The only law Youtube has to follow is the copyright law and it's doing it to the fullest extent possible in this instance.
This isn't MumboJumbo's fault in any way. But if there are samples and they're not cleared, there's a legal issue.
That does not - of course - give WC the right to steal 25% of the value of MJ's work just because YT says they can.