It's sloppy writing.
They should have mentioned it in an episode instead of leaving fans to fanwank it.
If they mentioned it, it would be as bad to the audience as the "Holodecks have their own power source" explanation was. If commerce planets, the exact process of making more torpedoes and how self-repair was done on the ship were all mentioned then the complaints would just be that there ARE Commerce Planets, that they can make more torpedoes and that they can fix damage.