Good point on the retainer, but there would still be costs, and whether or not the costs would be covered under the retainer agreement depends entirely on the substance of the retainer agreement.
Even “filing a simple request to dismiss” entails costs. Who researches the law and prepares the motion and brief? (billable, subject to the retainer issue) Do the defense attorneys not discuss strategy? (billable, subject to the retainer issue) Do the defense attorneys not talk to their client? (billable, subject to the retainer issue)
You say there “would be no travel and no appearances or arguments. Just simply file to dismiss.” What do you base that on? What if the court orders a hearing on the motion to dismiss, which is the usual course of action in the states I’ve practiced in. I have not looked through the court records for any of these cases, so I don’t know what documents were filed, appearances made, hearings held, etc., but I am pretty confident that something more happened than the defendant’s simply typing “I request a dismissal” on a piece of paper, mailing it to the court, and the court then dismissing the cases.