Seatbelts also make it easier to maintain control during a sudden sharp swerve, which can reduce the chances that a sudden swerve to avoid something in your lan

Seatbelts also make it easier to maintain control during a sudden sharp swerve, which can reduce the chances that a sudden swerve to avoid something in your lane will turn into an out of control excursion into other lanes. That's probably sufficient to justify a mandate for the driver and possibly front seat passengers (since a sharp swerve toward the passenger side could cause an unbelted front seat passenger to hit the driver).