I'm guessing that Carpenter simply didn't have enough money to pay Henstridge the going topless rate (and indeed, she deserves to be paid) since everything in this movie is done as cheaply as possible. On Mars, there's supposedly a train that transports people between mining colonies. The effect is so bad that at times, you can see the hand moving it along the track.

Carpenter is trying to create some Third World analogy here. Basically, humans are mining Mars for its resources when they unearth some freakish spirit that takes over their bodies and turns them into extras from "Night of the Living Dead." The freakish spirit does see fit to turn one human into Marilyn Manson, so that the extras can have some sort of leader. According to the general assumption made by Lt. Melanie Ballard (Natasha Henstridge), the rest of the Mars police force, and a criminal they're sent to bring back, James "Desolation" Williams (Ice Cube), this is sort of what humans deserve for strip-mining Mars.

The other members of the Mars police force include Bashira (Clea Duvall, in her first non-frustrated teen lesbian role), Jericho (Jason Stratham), Descanso (Liam Waite), and Commander Braddock (Pam Grier). While searching for Williams, they encounter Professor Whitlock (Joanna Cassidy). They have weapons to kill the possessed humans, but quickly discover they dislodge the spirit within and it just seeks out another human host. But back to the more central thesis: Natasha Henstridge in a movie about strip-mining. How could such a thing could not include a nude scene or two? Is there no God?