This is confusing to me. Does the car prevent a safety feature (setting the parking brake) from working when one doesn’t wear the seatbelt? Or was that a typo intended to mean that it prevents releasing the parking brake (and allowing the car to move) when one doesn’t wear the seatbelt?
This sentence is confusing but doesn't seem to make sense either way it can be read. What is the antecedent of "it's" in "while it's still on"? The parking brake or the seat belt?
Why should you not exit the car when the parking brake is set? Alternatively, how could you exit the car while your seat belt is still on?
Not letting you set the parking brake? That's also known as the emergency brake, you don't want to disable it while the car is moving.
Not letting you remove your seat belt? Obvious serious safety problem.
Not letting you open the door? Super obvious super serious safety problem.