There is no legal precedent requiring these services.
There is legal precedent for requiring captioning where I’m from and probably in the US as well. Practically every form of broadcasted video (and at least here, it is required of websites with video) has a legal requirement to provide captions. I don’t see how it would be difficult to apply that to music.
It being available on the free tier has almost no relevance to Spotify being a profit making entity that has to comply with the law. I’d be surprised if they don’t get in trouble for it legally. As pointed out elsewhere it’s paywalling an accessibility feature. Which seems like a great way to draw enough eyeballs to your bullshit and get legislation changes; assuming it doesn’t already violate it.