"An Open Letter to Hobbyists", 1976 (he was 20).

https://en.wikipedia.org/wiki/An_Open_Letter_to_Hobbyists

In short it says: I can't make you pay me, but I'm calling you a thief, so you should pay me anyway, or else I'll call you a thief again. Other people get paid, so I should be paid too because I like money.

Change "thief" to "free rider" and it's exactly what the shared source / post-open license folks are saying.