its friction, a really really high amount of friction.....
Not to mention many of the subscription sites (like newspapers) WILL NOT ACCEPT a prepaid card, or a virtual card - they flat refuse them. It has to be a card they can auto-bill forever. Not everyone does this, but enough do that its really rather bothersome friction.
The people they lose from not accepting pre-paid and virtual credit cards costs them less money then the people who forget they signed up for a auto-renewing subscription and pay for months/years without realizing it.