The whole point of a business (especially restaurant) using Groupon is to gain a repeat customer base. To that end, it would make more sense to give better than normal service to the 'cheap' Grouponers.

For a restaurant, what will cause people to come back is quality & service - so if you are outlaying cash (via discounts) to gain customers, you would think you would go out of your way to gain a repeat customer.

Perhaps this is more of an education for the wait staff, and perhaps the restaurant owners should realize the people using a coupon may not tip well, and make up some of the difference for the wait staff - and emphasis the importance of converting them through service & quality to be repeat customers.

Otherwise - why would the owners do the Groupon at all?