VAPA,
It is not quite as bad as you think. And the answer is in two parts -
Martinez-Fuerte for the stop/detention and
Mimms for exiting the car.
Part One: A stop/detention has to be legal. The
general rule is that PC or RS is required for a stop/detention, but
Martinez-Fuerte provides a
special rule for immigration checkpoints. The Court allows stops/detention at immigration checkpoints, without PC or RS, simply to help achieve the public policy goal of preventing illegal immigration. The only limit on a checkpoint stop/detention is that it must be "brief" (more later).
Part Two: Once a legal stop/detention has occurred,
Mimms kicks in.
Mimms is a
general rule that a driver can be ordered out of a car, totally at the police officer's discretion. There is no special provision in
Mimms, or any other case, about exiting a car at immigration checkpoints, so the general rule always applies.
Put the two parts together and you have the whole picture. A normal stop/detention requires PC or RS, so there is already a logical officer safety basis for making the driver exit the car. An immigration checkpoint stop/detention does not require PC or RS, so the officer safety basis for making the driver exit the car may not seem as strong. But
Mimms does not recognize any difference between the two types of stops and simply says: legal stop = exit the car.
Regarding "brief detention" at immigration checkpoints,
Martinez-Fuerte gives a bit of time for normal questioning, but requires PC, RS, or some other suspicion to detain someone for an extended time. At the checkpoint in
Martinez-Fuerte, the BP reported that the average time for a secondary stop was three to five minutes. The Court did not object to that amount of time, so "brief" must be at least three to five minutes. How much more time would exceed "brief" and possibly cause a detention to become illegal? The Supreme Court will tell us when they see it in a future case.