You would think that is the reason, and not the fact that their matching service can only appropriate value into the company if the two peers being matched can not effectively coordinate themselves?
That's plausible, except when you do it that way there's a really great chance you'll be screwed over. Airbnb can protect its users to some degree, it's up to you whether you want to use it or not.
Not in my experience. AirBnB holds all the money and they can choose to refund or not depending on the circumstances. Just try renting in another city with Craigslist. It's full of scams.
But yeah, they censor emails, phone numbers and such in order to prevent deals from being made off site. They don't do it to protect anyone.