If you look at their FAQs, the implication is that the restaurants report no shows, not bookings who turn up:
Help | bookarestaurant.com
No shows are linked to your account on the site so any attempt by the restaurants to report fake no-shows would probably get noticed fairly quickly - either by the restaurant patrons when they saw their account or by simple statistical analysis on behalf of the site - if 10% of no-shows is normal but you report 60%, questions might get asked!
So my theory:
1. Reservation is made - restaurant pays a fee to bookarestaurant.
2a. Customer dines - no further interaction, points are awarded.
2b. Customer no shows, no show is reported to site, customer's account is flagged, points award is halted, possibly a refund/credit is made by bookarestaurant to the restaurant.
So the restaurant has an incentive to record no-shows. If they don't it is their loss and no-one else's.
Maybe there's a 2c. to cover genuine cancellations.