You brought up a good point, so I went back to the original announcement of the Medium Partner Program and found this language:
If you’re in the Partner Program, you’ll earn money when subscribing members read your work.
Partner Program writers are paid based on how deeply Medium members read their work. As members read longer, writers earn more. In addition, we distribute a portion of each member’s subscription fee to the writers they read most each month.
This language has a subtle difference. It sounds like a portion of each member’s subscription fee is paid as a bonus to the most popular writers on the site each month, while the new announcement doesn’t say where that portion of subscription money is going.
As I wrote in another comment, it’s perfectly reasonable to take a commission of writer revenues for creating the platform. My objection is to their lack of transparency because it creates uncertainty over the rules of the game.
And gamification is the way modern online business create online addiction by their users.