I ditto Nathalie. Too many people are signing up to use Airbnb without knowing the basic information -- such as hosts who sign up and then ask, does the guest pay me when they arrive, or as in your case, the guest who thinks they are asking you a question, but has in fact booked a reservation.
I do think that Airbnb made it a little more difficult to simply contact a host and ask them a question, without submitting a reservation request. It used to be that the "contact host" button was up at the top of the page, and the request to book was not so prominent. But even so, there is information as one goes along that tells you that you have submitted a reservation request and that it will be confirmed if the host accepts it.
We as hosts want guests who read our listing----- so it's good that sometimes guests have to pay a "fine" if they dont' read all the info. It is a good way to drive home a lesson. I just hope the mistake isn't too costly.