I think you're right that they don't love it - and if I were in there shoes I'd have trouble keeping a pleasant demeanour if I was losing money from people being in my restaurant too!
This is opposed to the alternative of traditional advertising, where you're losing money on the people who aren't even in your restaurant.
These deals are reasonable to offer if your overhead is more than your incremental costs, for instance. Or if the look of a full restaurant (and thus the vibe) is more important than making money off of those initial customers.
The joke is groupon taking so much for brokering this. Groupon doesn't do anything to warrant $1 per coupon, let alone 1/4 the face value. If it takes off it'll be on ad-driven sites taking no or very little cut, and/or via the restaurants' websites and facebook pages directly.
