It’s a question of perspective. I agree with you about the free expression of ideas. But no writer expects anyone else to pay for his tools like pen, ink, pencils, sharpeners, books, phone, internet connection and so on. I see Medium as just another tool like those, and I don’t mind paying a small price for it, any more than I mind paying for my pencil.
Secondly, for ideas to be expressed freely, it needs a platform to be seen, and platforms need money to exist. So the price of not paying-to-write may be too high. What if Medium has to allow ads to survive, and turns into another ad cluttered site that tracks your every move, and sells your info to anyone who will pay for it. The writers who make Medium what it is today will disappear, Medium will die, and everyone loses.
I think a distant parallel is the music industry. Piracy was killing it till iTunes began selling songs for 99 cents, a price that must have been hard for musicians to digest. But in the long run, it encouraged people to buy music legally, and it worked out better for both musicians and their fans. So maybe a pay-to-write model will help Medium become self-sufficient, and eventually work out better for both writers and their fans.
I must add that I see paying-to-write as an evolving model where paying may be discontinued once Medium stabilises and figures out how to generate revenue in a way that is a win-win for Medium and its writers.