The catch seems to be twofold:
1. The voucher expires in a couple of months, so you have to use it withing a certain amount of time.
2. In the example they show, you give them $18, and they give you a voucher worth $50 at the named site. The catch here is that you need to spend at least $18 to break even (probably not a problem), and at least $50 to really get the deal you're after. And you'll likely spend more than that, plus the voucher usually doesn't cover shipping costs.
I only looked at the site briefly, but assuming it's legit and you're "that kind" of shopper, it is a good deal. We do similar things through restaurant.com and the local restaurant coalition's website, and there are "catches" there, too. (Expiration dates and there is usually a minimum purchase that does not include beverages, and sometimes restrictions on entrees, etc.) But overall, the catches aren't completely prohibitive (we don't drink much anyway, and aren't stuck to trying to only spend exactly $50 if the voucher is for $50).