Seconding Lady Lover's explanation.

Credit card fraud is mucho big time with RMT (stolen, falsified info, chargebacks, floating CCs). Where Click and Buy may have been a pain in the a$$, it's the price of security. ClickandBuy gave merchants a greater peace of mind that items or services purchased are purchased with legitimate funds. (validation requirements may have increased overtime due to fraud)

But seeing as the RMT suspects are present in the game, they must have found a hole in the defenses (most likely not with CaC).