The musician argument is less with Spotify than with labels, and how Spotify revenue is shared with artists by those labels. When Spotify income is treated like album-sale income (physical or download), artists share the money according to a royalty split, typically 10 percent. Licensing revenue, by contrast, is often set as a 50/50 split in artist contracts, and the Swedish complaints assert that Spotify distribution of music tracks should be a licensing scenario. The musicians union involved in this controversy is threatening action to remove artist repertoire from Spotify.
This Swedish page can be translated for quotes by the union head. The Guardian has an English write-up here.