You can be pulled over anywhere at any time without a court order or even the permission of an officer's supervisor. All it takes is a reasonable suspicion that you've violated any of hundreds of fiddly traffic laws. People generally don't have a problem with this; you learned about this when you were 15 and in the classroom part of driver's ed.
There are places where you can be pulled over for no reason, by default, because everyone is being pulled over. These are usually "sobriety checkpoints", but sometimes they come right out and call them "compliance checkpoints". In some states, they're lawful; in others, their lawfulness is more questionable.
I have a problem with checkpoints. Lots of people do. So there's a bright line: you can pull some people over (you hopefully have a reason, but we accept the notion that nobody has time to make sure and then if you're an officer and abuse that privilege there is a process for disciplining you). You should not be pulled everyone over.
There's a slope, I agree, but I dispute that it is as slippery as you say.
There are places where you can be pulled over for no reason, by default, because everyone is being pulled over.
This is the case in Canada, and I don't know anyone who has a problem with it. The police can't pull everyone over and demand that everyone provides a breath sample for alcohol analysis -- but people accept that if you've got 10 police officers stopping cars, it's a heck of a lot safer to stop everybody than to have them jumping into and out of traffic picking out individual cars.
