But this is criminal! They'll hang them for it!

Not necessarily . . . As you know there is a sort of undeclared war going on between certain colonies and Earth. With enough money flowing in their accounts the mass media will gladly proclaim it a terrorist attack. The chief and his men will simply hide inside those "harvest hangars" and when all is over they'll tell the rescue squads some made-up story.

So that's why they're making those hangars so deep !

Yes, but that sort of refuge won't protect them from a dust mine and the experts know this. That's why they're taking away inessential agents.