They're trying to position themselves as Yelp of coupons. They snatch coupons that people use (for example 90% off special coupons meant for specific people, that just happened to have the extension installed), then they have a partner program for companies that allows the partners to control what coupons honey can apply on their site.
You don't need to join it, but then your customers get automagically the high value coupons applied, but if you pay them $XX per month, you can limit what gets applied.
Honey is a parasite stealing from other parasites.