Sometimes the backup forces just need to get in there and do the primary job.

The way to avoid this is to not give the PCs any backup forces. Yeah, that's pretty much the only way.

Transcript

Lando: I read an explosion on Endor! Admiral, any word from Han and the others?
Admiral Ackbar: The Imperials must have sabotaged their own facility to confine them to the surface.
C-3PO: Um... something like that. I'm afraid we've failed our part of the mission.
C-3PO: Artoo's out of commission. We can't hack a fighter to get you through the force field.
Lando: Wait, this is strange. There was a huge databurst beamed from the surface to the Peace Moon just before the explosion.
C-3PO: Record that data!
Lando: And... the force field is down! We can attack!
Admiral Ackbar: But we can't just hit the exhaust port - they've actually carried out design reviews this time and exposed heat radiating fins.
Han: They can't attack! We're the PCs!
Leia: Yeah, we were never given a fifth chance to contribute.