The fraud claim must be that the person who initiated the transfer (who interacted only with Dwolla) wasn't authorized to perform the transfer by the person who owned the funds (who interacted only with Dwolla). Who does the merchant follow up with? Two people he interacted with only through Dwolla?
I don't see anything in their user agreement that says that can reverse transactions. Dwolla has gone, in my opinion, from useful to useless. I'll raise my opinion of them a bit if I see what information they provided when they reversed a transaction. If the released literally all information they had from both contacts, I'll revise my opinion a bit.