The sensors are always in the passenger compartment, as its the rate of decelleration of the seats that they need to know...if you are decelllerating too fast, you get hurt, like hitting a wall, etc.

If the seats are slowing down at a reasonable rate, because the OTHER car is crumpling like an aluminum can, the deer is beeing venison pureed, the hood and crumple zones are folding like they're supposed to in a severe enough crash, etc.....then the bags don't go off, because the sensors are not slamming to a halt too fast, etc.