How heavy is your laptop bag?  According to my passenger airbag sensors my latptop bag is at least as heavy as a small child.

The way I understand the system is that if the passenger is below a certain combination of height and weight, then the airbag is turned off and the seatbelt light is not triggered, but if you are a little heavier and/or taller then the seatbelt light is triggered.

The point of this is apparently to prevent injuries to small or under-weight passengers by a deploying airbag.

Awful nice of my truck to want my laptop to be secured by a seatbelt but not injured by the airbag.