Big tubes can never have processing (eg the 1click idea), there is too much content that is not allowed by the card schemes. A lot of big tubes got burned already when they had the premium on a subdomain of their main domain.
That's why it's made so clear that you are leaving the site and go to another site.
Most of the 'premium' versions of tubes are nothing more than paysites with DVD content and AC whitelabels/channels. They have nothing to do with the content on the tubes itself.
When you click watch HD or see on Premium the chance that the video is actually there after you pay is very small. It's just another way to get people to signup for recurring memberships.