I don't have a problem with this - they are traps after all. I believe each round represents about a week in real terms, so setting traps to be triggered even when you are not present (I Am Legend anyone?) seems quite thematic.
Okay, I'll give you that, I mean the survivor is 'there' if the place is overrun, but the survivor could reasonably shelter from his own team's bomb. What a bout the bandits? Maybe the survivors put up an "Achtung Minen!" for the benefit of their own team and the bandits read it.
What about targeting bandits with a sniper rifle or grenade launcher, or from the treehouse?