I said all cars, but that may not be true. I shoulda' said, many cars. The other factor may be the advanced, or 2-stage, airbags which are capable of reduced force, and by sensing weight in the seat can tailor deployment for kids/small adults, thereby obviating the need to disable the airbag completely.

 

When they enacted the law the required baby and booster seats, they also included (or enacted shortly there-after) the requirement that 2 seaters have cut-offs because that's where you would have to have a baby seat. As far as I know, in a car with a back seat, the cut-off and multi-stage airbags are not required by law, just a "feature" than many are now providing.

Either way, its always a good idea to check exactly what the car has/does before putting a child of any size in the front seat.