But the point is, once they were killed, they wouldn't have been "staying" there anymore. Any stay would've been quite temporary, and thus effectively wouldn't have counted. It's fundamentally different from the Cardassian DMZ situation, since that was a choice between leaving and living under oppression and harassment designed to force you to leave. This was a choice between leaving and dying, which I don't see as any choice at all.
I think the point is that they can choose to go with the Enterprise or choose to stay on the planet. Yes, staying would mean their certain deaths, but the colonists were convinced (however wrongly) that their deaths were not certain if they remained on the planet.