Presumably musicians that want this release their work under a license that allows it.
Just kidding, those three companies have been granted a state guaranteed monopoly over enough of human culture to be able to bully other entities like YouTube, Spotify or whatever, into accepting onerous terms.. (think youtube automatically (and legally) stealing ad money and giving it to these companies, for a video of a person playing historical scores on a recorder. Or Spotify accepting that the money you pay them will get rerouted to famous artists you have never listened to)
Vast majority of musicians have no clue about licensing or copyrights or/and do not care, be it permissive public ones or strict ones.
In this case I’d regard musician as someone who derives significant portion of their income from making or playing music or at least puts in a significant time into becoming the above (practice, learning and playing).
Same goes for platforms with automated copyright checking. Enough music makers get caught up in labels that don't get this either, that you'll almost always have three or four tracks striked out of your recording making it pointless to upload.
There has never been any money in live online DJing, it's done for fun and the love of it. So it just feels like petty fun policing.