When I signed up for the United Airlines credit card through Chase Bank to get 20,000 frequent flyer miles, I also got a $20 check which, upon deposit, would enroll me in Chase's credit protector program. Most credit card companies seem to have some sort of credit protector program, and many offer a cash incentive for signing up because they know that in most cases, they will quickly make their money back, and then some. I knew that I was done making purchases on my United card and that I would soon be cancelling it, so I could essentially take the $20 and run. And I did.
The way the program is supposed to work though is that you deposit the check, which signs you up for your no-risk, free 30-day trial, then you forget that you're enrolled, and you start getting charged a small monthly fee based on your total balance, usually something like 89 cents for each $100 of your ending monthly balance. Anyone who doesn't closely review all the charges on their monthly statement could easily overlook this small extra charge which, over time and multiplied by thousands of people, adds up to lots of free money for credit card companies.
The supposed benefit of the program is that if you have a major life event such as having a child or losing your job, you can put off paying the balance on your card without penalty. I've never tried to do this, but to be honest, I have very little faith in credit card companies and I wouldn't trust them to follow through on their end of the bargain--I'd expect them to find something in the fine print that they would use as an excuse to deny the grace period. On the other hand, I might be wrong about that--the other possibility is that they'd be happy to allow the grace period with the hopes that I'd rack up more than I'd be able to pay off when the grace period ended, allowing the credit card company to levy all sorts of ridiculous interest fees.
At any rate, I'm not sure how difficult it would be to cancel the credit protector program after the free trial ended, so I would never sign up for it with a card I was planning to keep unless the check was for significantly more than $20. But I was able to make $40 free and clear by depositing checks from two different credit protector programs for cards that I was about to cancel, making it an easy source of free money. When I cancelled my cards, my enrollment in credit protector was automatically cancelled, but the incentive was mine to keep.