Spaceports are only for resource/etc points generated by colonies. You can remote mine a system with two dozen enemy spaceports and still the resources will go to your stockpiles.
And btw, how else would you think P&N Nomad players get their resources?
__________________
O'Neill: I have something I want to confess you. The name's not Kirk. It's Skywalker. Luke Skywalker. -Stargate SG1