They have a license to play music, and they pay for the songs at a rate that the labels charge for. If artists do not get enough money from streaming then their label should negotiate more from Spotify. If they are already taking 80% of Spotifys income then that means that either they aren't charging Spotify enough, and Spotify should charge users more, or they are taking a larger chunk and deflecting blame.

With them using the RIAA for the longest time as a distraction from it actually being Sony et al that was suing it's customers, I would not be surprised if this is misguided blame again.

But again, if artists do not get enough money from streaming music, then that is not Spotifys fault. It has paid for a product that it is reselling. You don't then get to bitch that you sold someone something for below the cost that you wanted to.