the people who actually paint and make Jeff Koon's art, for example, do not have any expectation of ownership. And if you are the artist and are upfront with what the working conditions will be, then your workers can either agree or move on to something else. Like if I think the melody and lyrics are what constitute songwriting, then I would want all of my studio musicians and producers to know upfront that I think adding chord inversions, riffs, or spicing up the arrangement would fall under mechanical rights or something (depending on how much of those contributions end up on the actual recording).

Moreover, an artist might be open to suggestions on changing up words, but that doesn't mean you're a songwriter. It would be like if an author included all of the people who gave edits and suggestions on their manuscript in the final list of authors. Some people like Kanye will credit everyone in the room during studio sessions, while others wil argue that without them overseeing their project then none of those other contributions would happen without your original demos or outlines of what you ultimately want.

In Ms. Hill's case, her record label created really shitty contracts that the musicians she was working with had signed (because most of them didnt know better). The record label then pushed her to settle out of court to protect their own control over her master recording. Most professional session musicians routinely contribute crucial parts of songs that they never get credited for. If she had taken this to court AND these musicians really did sign away their contributions, then there is a good chance she would have won. But I am not an entertainment lawyer. And I also do not know her or these musicians. So maybe someone else on here would be better able to speak on this stuff.