I think the driver's airbag is always on. That of the passenger, however is turned on or off based on said passenger's weight. I think it's a way of ensuring that if there is a child, or as you mentioned, no-one in the seat, the airbag doesn't deploy.

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I'll drink your poison if you fill the cup. You make me crazy, baby, don't let up.
Break Free, DMB 2006