1. An overload of customers they cannot handle especially right before the coupon is set to expire from it's inflated value.
There isn't a business in this world that has ever (seriously) complained about having too many customers.
2. Coupons are generally used to build customer bases. Because the customers are loyal to Groupon, they aren't interested in the restaurant itself, just the deal they got from Groupon. Thus the advertisement does nothing but give them even less money to make and little to no repeat business.
Anything that gets a customer in the door is a good thing. Every customer that comes through the door is a customer who might become a repeat customer. You could argue over the relative effectiveness of Groupon versus other advertising methods (and advertising is exactly what this is), but to say that Groupon in and of itself has no value (in fact, you imply that it has NEGATIVE value) to a business is ridiculous.