The Mechadusters in the Shopping District are non-hostile and cannot be attacked.
During the final battle in Carltron's Lair, a Mechaduster will spawn between enemy waves to clean up the remains of the enemies, and then leave. They will not attack. The player can destroy them, but this is not advised; killing a Mechaduster will cause Death Spiders to accompany them later on for protection, and the Death Spiders are quite dangerous.
The player doesn't get much experience or money for fighting the Mechaduster, further discouraging doing so.