Notice that morality doesn't have to have anything to do with this argument. As a matter of pragmatism, the operator of a Silk Road-type site is almost certain to harbor a conviction that he can do so without getting caught (regardless of whether the authorities have morality on their side).

The same same goes for stealing from your customers: of course you won't get caught, what are your victims going to do? File a police report that the money they gave to a contraband site to presumably purchase contraband was stolen? The same conviction that makes it possible for them to operate Silk Road 2 makes it possible for them to steal from their customers.