Pretty much the same way that Nigerian Prince who wants to offer you money does! The breach from scammers is not due from signing up with companies, it's from the usual sources, clicking on something, signing up for a mailing list, forum, freebie, etc. I use a dedicated email for mystery shopping and while I've definitely received my share of scam mystery shopping emails, I don't think I've ever gotten one to that address.
Also, although there is the occasional legitimate referral from another MSC or scheduler, solicitation is relatively infrequent. The sender's email address and domain is usually a clue as well but when in doubt, check the forum!