The RFC 3101 (OSPF NSSA Option) states:
In addition, an NSSA border router should originate a default LSA (IP network is 0.0.0.0/0) into the NSSA. Default routes are necessary because NSSAs do not receive full routing information and must have a default route in order to route to AS-external destinations.
I am pretty sure that IOS inserted the type-7 default route into an NSSA area when the NSSA feature was introduced.
However, at least in some Cisco IOS 12.x releases, you have to configure the type-7 default route origination explicitly with the area nssa default-information-originate router configuration command, in which you can also specify the route metric and metric type (N1 or N2). Entering the area xx nssa without the default-information-originate keyword will result in an NSSA area with no connectivity to external destinations redistributed into OSPF in other areas.