Well, the Court has had a lot of supremely idiotic rulings in this area, so it's nice to see them get one right and more than a bit surprising, especially to see a unanimous opinion. If this were a factory and police were guarding the only exits, then workers would feel free to leave during the middle of the work day according to the Court. And while the Court now acknowledges that passengers would not feel free to leave during a stop, it's routinely held that passengers would feel free to say "no" to a police search during a stop. So you might think this ruling was bloody obvious, and you'd be right, but the Supreme Court would disagree and call this a hard case.
I would think that since the passenger was not guilty of any violation of the law that they would be exempt from any searches.
When I was younger if a the car I was in got stopped and I was a passenger, I would never show id if asked. I don't have to have a drivers licsense to be a passenger. And if i don't have id on me, I don't have id. Its on the cop to push it if he wants. His main concern should be the driver unless I do something suspicious or illegal during the stop. Like if you had kids in the back seat. They don't have to show id.