There is no reason for conspiracy theories. Spotify simply made a bet in being able to be more than a commodified middleman, which has not yet (and may not) pan out.

Warner/Sony/Universal music groups are also not opaque, they are all publicly traded companies, just like Spotify. It just so happens that they have more negotiating power than Spotify, so they can dictate more favorable terms.

Also, artists are free to make deals directly with Spotify/Google/Amazon/Apple if they want to bypass the record labels.