Roughly speaking you're right, but there is already a demand for Onions, and there's no need to manufacture demand for "onions sold by Groupon" because Groupon

Roughly speaking you're right, but there is already a demand for Onions, and there's no need to manufacture demand for "onions sold by Groupon" because Groupon don't want to keep selling onions. It's as piece of marketing by Groupon, at the end of the day, and a reasonably clever one, hence it's on HN.