There are several important reasons for why it is designed so (and why other major reservation websites like Airbn, Homeaway etc. do the same)

– First of all, we want to avoid spam to our users. Allowing only register users to communicate on our platform we will effectively filter out most automated bots and spammers. 

- Secondly, we must have a users email to replies and notification messages, like booking confirmation, invoices or more important cancellation messages?

– Thirdly, Guest most likely would like to get in touch with the Host? Host contact details are available only after a confirmed booked and only for the user who makes that booking. 

These are just some of the issues that would happen without registration.

Also, communication solely on the plafrom between 2 registered users is the only way for admin to check the communication and resolve any issues there might be between the parties.