Hi John,
I think there is a good reason for this. Take a look at this post first:
Now imagine that the ABR and the ASBR both advertise a default route, when this is the case, OSPF will never use the default route from the ASBR since information of inter-area will always be used over NSSA external routes.
Cisco probably disabled the default route on the ABR for this reason. There’s no need to do this for stub or totally stub areas since they don’t support an ASBR.
Rene