I see where you're coming from, but do these require you to register? Do they need a billing address? Whats stopping someone from just getting a new one?
Censorship need not be targeted at the consumer/buyer, it can be targeted at the merchant/seller too. Even though you can obtain such cards anonymously, the fact that they still utilize those corporate payment networks means those corporations can block all transactions being sent to targeted merchants without needing to know the identity of the customers.