Occupancy detection does not preclude selective airbag deployment. While the force from an impact is a vector (magnitude + direction), airbag sensors are linear accelerometers. They cannot figure out which direction the force came from. If one gets T-boned, even the front-mounted sensor experiences such rapid deceleration that it signals the airbag module to deploy. A single or two (one redundant) sensor doesn't know how or where the car got hit -- just that it got hit.
I suppose if you have multiple airbag (collision) sensors throughout a vehicle, a DSP in the airbag control module can figure out where the collision came from. Better yet, add input from other sensors such as BSM (a radar) or cameras to assist in determining point of impact.
In the end, the airbag system is what's known as ASIL level D -- the severest of all automotive safety critical systems according to ISO 26262. Human life depends on it. It cannot fail, so safest to deploy all the airbags (except front passenger) and not have to make decisions that could affect passengers' lives. Airbags also need to deploy really fast, something like 50ms.