[giantg2]

So what's the difference between a front seat without the airbag on and back seat?

My guess is that it's really just automakers CYA so they don't have to treat it as a safety critical system. There's really no reason to have that sensor then.

[kube-system]

It is part of a safety system, but the other way around. One of the purposes is to make an annoying dinging sound when the passenger does not put their seat belt on.

[giantg2]

Eh, if one doesn't want to wear a seat belt, then don't. Pretty annoying feature if you're using it as a farm truck.