They would lose, assuming that they were not popular enough to fill seats with walk ins, which wouldn't be a restaurant that necessitates purchasing a reservation for anyway.
Even if they were popular enough to fill the seats they still lose, as they still lose those employees spending time on the phone to take the initial arbitrage reservation & then they lose time on the phone once again when the employee takes the arbitrage reservation cancellation call.
Yes, and they would lose this time with the man who books 3 restaurants because he doesn't know which one his girlfriend wants to dine at for their anniversary, 10 fake reservations from a competitor trying to block out a weekend night, etc.
Managing reservations and paying employees to man phones is part of the operating cost of a restaurant. Not to mention, this could be automated with software that ties customers to credit cards and reservations.