They do if they want to get them away from Groupon.

I have e-commerce customers who love Amazon Merchant because it's cheaper to acquire customers than SEO or buying mailing lists. But nothing pisses them off more than a customer who comes through Amazon a second time, costing them all their profit. They need to save the customer info in order to make the second sale a direct sale.

The same dynamic works with Groupon, except in person. If you don't follow up with them directly, there's a good chance they'll just wait for the next Groupon promotion, whether it's from you or your competitor. There's anecdotal evidence that this is already happening. A fresh coupon in your in box every day can get awfully addicting.

But for lots of retailers like salons, specialty shops, services, even restaurants, Groupon can be an effective way of building their customer book. When you call them back yourself, you have a much higher probability of converting them into a permanent customer.