They could be making money through affiliate marketing. Who knows, perhaps for each "shopper" who signs up for these services under the auspices of doing a mystery shop, they receive a commission. Sometimes affiliates make money for referrals to sites that don't charge users anything. Just gathering peoples' demographic information is lucrative in itself and some publishers will pay affiliates just for getting people to sign up. Eyeballs equal money.
Remember Michigan Bulb Company? They didn't make their money from selling flowers, but from amassing huge customer databases.
I'm not saying that's what this company does, because I've not looked at their privacy policy or IC agreement. I'm just saying it's possible and worth looking into.