Hi,
The "to" statement in the policy is a matching condition:
> from Conditions to match the source of a route> then Actions to take if 'from' and 'to' conditions match> to Conditions to match the destination of a route
I am not sure the external route can be exported to selected areas in OSPF, because external routes have an interarea flooding scope, hence policies are applied globally and external routes are exported to all areas under ospf. Statement "to area 0.0.0.23" is not a matching condition I suppose, hence 0/0 is not exported when the statement is present.
You could use an import filter on R3's OSPF to reject the default route from being installed in the routing table. However, this may not be a scalable solution if you have multiple neighbors in area 0.0.0.18 because of the LSA flooding.
Cheers,
Ashvin