"Credits" are in some cases more insidious than being sent physical goods, because they're less likely to be noticed. And you'd be surprised how slowly most people realize they've been subscribed to a service -- especially if the billing for that service is under a fairly innocuous-sounding or obscure name (as is often the case).

Paying "for one item in the first cycle and then cancel[ing]" is probably an exceptional case. I'd be willing to bet that it takes most users two, maybe even three or more cycles to notice and then break the subscription.