I'm a bit confused by the reasoning here. It's clear that light users are subsidizing heavy users, but why are we assuming that light users prefer indie bands and heavy users prefer pop music?
There is also the idea that some "users" are businesses playing background music all day, and that they are more likely to all choose the same pop songs (which the most people are familiar with) and to play them all day everyday. Unlike a normal user who typically listens to a wider variety and for fewer hours a day.
The article explained this - because heavy users are playing music 24/7 at gyms, etc., and never play the smaller groups or genres that only some individuals like (and would like to support).
I can't imagine it's being done illegally regularly. It would be way to easy to identify by stream behavior.
I think it depends on local copyright laws, but I think in most places you buy a license to play music publicly and with that license you are allowed to play any music legally acquired.
So basically, my dad, who listens to 60s and 70s music all day on Spotify is mostly compensating top-40 stuff. In the olden days of FM, his listening habits were paid for by the sponsors of the oldies station.