On-line Coupons: Buyer Beware

As you are probably aware, online coupon sites like Groupon and Restaurant.com have proliferated in the past year or so. It's easy to see why - there are some great deals to be had out there. And if a restaurant that you frequent is advertised, why not save some money on your next visit?
Be warned, though. I recently purchased a $25 coupon from my local pizzeria, for a price of $15. Not until I went to pick up our order of a sheet pizza and two dozen wings on a Sunday afternoon did I discover that the pizzeria does not honor these coupons on weekends.
Nowhere on the coupon is there any mention of a no-weekends policy. And while we neglected to mention the coupon when ordering, there is no mention of a policy about that either. The only restrictions noted on the coupon were the $35 minimum purchase requirement (which we met) and that it is valid for dine-in only. Now this was a takeout order, but that was not the reason given us for refusing to honor the coupon. The stated reason was that it was a Sunday, so even if we'd ordered it for dine-in, they would not have honored the coupon.
I later spoke to the owner, who told me, in effect, that he didn't know what he was signing up for when he agreed to this deal, and that he's cancelling his participation in this service (which apparently hasn't taken effect yet, since his pizzeria is still listed on their site). Although he assured me that he understood my being upset (gee thanks), his only substantive response was to say that he would honor the coupon on a weekday (which means we'd have to put in another $35 order), and to suggest that I contact the online service to seek a refund (yeah, good luck with that).
In short, I'm screwed. And while I don't blame the online service for what happened - it's not their fault if this guy didn't read his contract closely, or if he refuses to honor his own coupons - I'm not too inclined to buy any more of these things. If the business owner refuses to honor it, there's not much you can do about it. At the very least, I'd advise you to check with the business itself before you go shelling out money for one of these coupons.
Pizza Guy Update, July 29, 2011: I contacted restaurant.com and told them what happened and they did give me a credit good for a new $25 certificate. That's good customer service.