The original post talked about this causing a lack of discovery for the new music. If the streamer doesn't pay and just uses non-copyrighted music no harm done to the streamer. It's still great fun. But the users don't get exposed to potentially interesting music content.
Why should the streamer pay for the privilege of being basically an advertisement platform for a commercial product? And they don't, and thus there is less discovery. Hence what is meant by the music industry shooting itself in the foot.
I personally listen mostly to music I listened as a teenager and early 20's. Sometimes I learn of something new and nice from my friends, but otherwise I find discovering new music quite hard.
A new game comes out: Tons of reviews and letsplays and whatnot. Looks interesting, I'll buy. New music comes out: Its advertised maybe in Spotify but that's it. Making a review or analysis is not possible (without paying extra), so I can't encounter it. And thus I won't listen it.