The spanning-tree portfast disable command explicitly disables PortFast for the given port. The configuration line appears in the running-configuration because this is not a default option.
The no spanning-tree portfast command implicitly enables PortFast if these conditions are met:
If PortFast is not configured globally, the no spanning-tree portfast command is equivalent to the spanning-tree portfast disable command.
For more information, refer to the spanning-tree portfast (interface configuration mode) section of Catalyst 6500 Series Cisco IOS Command Reference, 12.1 E.