Agreed. If you book a girl and she shows up, then you cancel for any reason that is not the girls fault, at the very least you owe a kill fee.
She made the effort and expended the cost to get there and lost out on the possibility of work elsewhere. She deserves to be compensated.

And, if treated fairly, most agencies would cancel their agency fees in a situation like this.

This is not the kind of reputation I would want to create for my production company. No agency would book with you and you'd be left with craigslist as your only source of performers.