Transmitting everything ClearQAM is stupid. They would have to structure their system so that no two packages used the same channel, and then install hardware filters for each subscriber at the pole. It would take weeks to get a tech out to change your subscription, and it would be a massive, pointless hardware overhead.
There's no problem with conditional access systems, and there's no problem with CableCard when it is used as a simple conditional access system. The tuner feeds encrypted data into the card, and the card feeds unencrypted data back to the system. No expensive hardware filters, easy management of subscriptions, free and clear access to everything you're paying for. Everyone is happy, except for the pirates wanting to steal cable, and the content providers who don't want subscribers to have free and clear access to their rightfully purchased content.
CableCard is only a problem when it actually invokes its DRM capabilities, and because of the expensive certification process it requires of authorized hardware to uphold its DRM capabilities.