In short, the credit card was not meant to be used online. Many of the credit card scams are occurring at adult websites. Further many of these adult sites are owned by the same operators who perpetrate the same scam over and over again at different sites. Many scammers are never caught or noticed because they cram small charges to credit cards which people do not notice. Alternatively, many issuers of credit cards fail to stop unauthorized crammed charges month after month because the amounts are small and because of the weaknesses in their systems.
Scammers know which credit card issuers have weak systems.
Many people who are scammed at adult sites are afraid to report such scams because of where it occurred. The scammer then continues his or her scam. Be suspicious of any site that requires your credit card number for age verification. This usually is a common scam at adult sites. There is no reason to give your credit card number for a free trial period.