I agree there's nothing wrong with it. But does it work? Does Medium make money? It might be, despite the whining, they have it right in that they don't let an expectation of "this is free" ever develop.

That strategy might be better than writing a long-ish blog entry as a teaser, where the reader thinks they are going to get their information for free, only to be told "Psyche! You've actually got to pay for it!"