> This plays music from sources that did have a right to give you a copy (e.g. youtube).

The distinction being that any random copy of something on YouTube might be there not because the rightsholder explicitly wants it there, but merely because the rightsholder 1. doesn't work with a big label that participates in the YouTube DMCA content fingerprinting program, and 2. doesn't have the resources to stay on top of every unauthorized upload of their work on their own (or perhaps doesn't even have awareness that anyone is doing such.)

In other words, while YouTube Music (the music and music-video hosting and proxied-leadgen service) is essentially as authorized as MTV, YouTube (the user video hosting service, where a video might just so happen to be music + a static screen/lyrics) is a definite "grey market" for music. There's plenty of legit music there (e.g. live performances by the musicians themselves) but also plenty of freebooted content (...of mostly non-RIAA musicians, sure; but what of it?)

And in my mind, that makes YouTube (again, not YT Music, YT-the-video-host — yes, they're collapsed together at the UI level, but crucially, not at the API level!) not really any different from your average BT tracker, in terms of its ability to guarantee authorized-ness of what it hosts; which is why I think the comparison between "an app that plays videos it finds on torrent trackers" (Popcorn Time) and "an app that plays music it finds on YouTube" (Nuclear) is valud.

Eh, the distinctions being that

- With YouTube, unlike with torrenting, you aren't distributing the files.

- You have no reason to believe that YouTube doesn't have an entirely valid license - while you do with torrents. YouTube takes reasonable (though not foolproof) steps to attempt to ensure that. Asserting you can't use YouTube because someone might have uploaded a copyright infringing work would lead to the conclusion that you can't browse the rest of the public internet for the same reason.

- YouTube complies with the DMCA for whatever the safe harbor provisions are worth (under US law).

If it's a grey market, it's a very light-grey market.

This is a theoretical distinction. Most currently popular mainstream music has an authorized upload on YouTube.