At Groupon merchants get paid for unredeemed vouchers (not all at once but staggered).
For the human psychology of a business owner of say a restaurant that's terribly, as this means every time someone walks in, that's $25 of revenue and say $5 of profit walking in for your business and its employees to extract, but a guy walking in with a voucher is just $20 of costs.
Even if the voucher was bought for $25, that feels like money 'earned' weeks ago, and anyone who actually makes use of the voucher feels like it's costing you $20 even though it'd constitute the same $5 profit as a normal customer.
Combine that with the fact the vouchers in this hypothetical example wouldn't actually be priced $25, but rather discounted to $15 or $19 which has to be shared with Groupon, and the fact those who're use Groupon (discount seekers) not always but on average tip less, really kills the mood for typical Groupon's merchants: struggling business owners who don't sit in an office and look at customers on a purely rational level and see them as just numbers.
So I agree, it's a human psychology flaw in the business model. It's not unsurmountable, if Groupon runs a solid Merchant Centre full of 3 minute videos for merchants on Groupon's research that shows how their service affects the success of a Groupon deal generating repeat business, how instructing your employees to say x y z words when a Groupon member comes in etc, you could go a long way to mitigating this.
Google for example runs a clever incentive structure where the quality/relevancy of the ad plays a role in your ranking vs other merchants, and so the better & more relevant your ad, the less you have to pay for your add to be more visible. This means some merchants who build relevant ads actually pay less to be on the nr 1 spot in the search ad rankings, than companies who pay more but offer shittier ads. And they run a whole merchant centre with tons of videos to explain this whole process and what you can do to improve it. [0] This improves the experience for users by rewarding merchant's quality control, which benefits them, too. Groupon should run something similar which they probably already do in some way but I'm not very familiar.
Another solution is more controversial and hard to sell to merchants, which is to not pay merchants for vouchers that go unused and expire. This way a merchant can't be 'hopeful' that a Groupon user doesn't come in, because he doesn't earn any money when a person buys a voucher and doesn't use it before it expires (like I've had happen myself a few times.) Every groupon user who comes in is one he'd otherwise not get paid for, and so it flips the story around, every time a person comes in he gets paid by redeeming the voucher. Right now he gets paid regardless and people coming in are just costs, so obviously a merchant isn't thrilled to see groupon users come in today.
[0] https://www.youtube.com/watch?v=PjOHTFRaBWA