That’s an odd take. You are conflating completely different things. It costs close to nothing for artists to get their music on Spotify ($25 a year for several releases if I remember correctly).

There is no promise from Spotify that they will guarantee discoverability. Tens of millions of releases are uploaded to Spotify annually, how do you surface that to users?

Paying for promotions is ads. Artists (or their labels) are fully responsible for buying ads. They are advertisers. They spend money because they want to become more popular.

Users, on the other hand, buy a paid product. When they become the product it’s not by choice and it’s not because that improves user’s experience.