> if the musicians didn't want folks to listen on YouTube then they wouldn't put their music there.

You are giving musicians way too much agency here.

For most musicians on YouTube, they're there because their label and/or their manager wants them there. A big part of why musicians sign with labels and have managers is specifically because they don't have the inclination or expertise to micromanage this stuff.

I'm sure in many cases, musicians would rather not be on YouTube at all, but their already-signed nebulously-worded contract with the label doesn't give them any control over that.

In a world where everyone is a perfectly spherical rational actor in a libertarian vacuum, your argument would make more sense. But we don't live in that world. We live in a world filled with primates doing the best they can with the weird cognitive capabilities nature gave them and trying to get through each day with a little joy and dignity still intact.