This is one example. Airbnb isn't actually renting the place out to you (the landlord is). They discourage 2-way communication because what will end up happen

This is one example. Airbnb isn't actually renting the place out to you (the landlord is). They discourage 2-way communication because what will end up happening is that the user will try to make a deal outside of Airbnb, get screwed, and then come crying to Airbnb for recourse (and it will make them look bad and discourage future customers). They also want to keep their clients.