When you spend $10 or whatever on streaming, you'd expect $7 to be split between the artists you listen to, but it's not. Instead almost all of it goes to the big artists anyway because in the big streams pool the small artists you listen to have a tiny proportion of listens, even if you exclusively listened to them.
This gets worse when you consider all those shops/cafes/etc with Spotify's top charts playing on repeat. They may be a small percentage of users, but they're an outsized proportion of streams, and almost exclusively big artists.
This is the way it is because big artists have big name labels, and those labels are the only ones who can negotiate, so the deal ends up favoring the big artists/labels.
This is definitely not how most consumers would expect it to all work, and therefore "unfair" seems like a reasonable term to use.