The trouble is that, if you use any buyer protections available to you outside of the platform, you risk being banned for life. That to me seems unfair.

For your case where they wanted evidence - what would have happened if you had no evidence, and ended up issuing a chargeback for "Services not provided? You would have been in the right, but probably would have been banned for life from Airbnb's system. The lack of competition and monopolistic nature makes this seem hostile to consumers.