People are allowed to create for free if they want to. Medium definitely depends on writers who are willing to write for little to no money. I've been doing that myself. Unfortunately, yes, it does make it harder for all of us to make a living as writers. But there's no rule saying anyone can't work for free.
I just think we writers should be able to ask for what we are worth and not feel like we have to give our work away just to have it be seen by people. Creative work is valuable, and I agree with Benjamin that Medium could do a better job of compensating its writers.