What does it cost an artist to run a stream? I thought Spotify hosted the content. Do you mean it costs money to run the bots that listen to the stream and get the artist recommended and heard by more people?
Making it expensive to operate an effective bot farm is part of the way that spotify tries to discourage botting. Spotify's bot detectors are good enough that it's not lucrative to just make a bunch of bots to give yourself streams for the free royalty money, but apparently not yet quite so good that it's not feasible to use bots to turn a large amount of dirty money into a lesser but still substantial amount of clean royalty money by way of funding a bot farm.
Getting out, say, half of what you put in is a losing prospect for somebody who wants a money printer, but might be acceptable to a money launderer.