There is actually the capability to do an instant transfer over visa and mastercard networks as well, but it is somewhat expensive (>0.20) -- the pin debit networks are smaller and more willing to negotiate this sort of thing, and since most debit cards are able to process on more than one pin debit network you can get 100% coverage with just a couple of the pin debit networks playing ball.

That they are not allowing this for credit cards indicates that they are using the pin debit networks.