Basically, officers were doing what they wanted, and just sort of fictionally claiming that there was probable cause.
So in this case, the USSC decides to do something about the situation. People expected there to be gradations of stops and frisks that would be declared not to violate 4A.
If an officer limits an individual's autonomy by asserting authority, that's a seizure, and 4A is implicated.
If there's reason to believe that a crime has occurred, and that a person has information that would be helpful (because of particularized articulable facts from which you can reasonably infer this). OK, this gives you the authority to stop someone for additional discussion. If you've stopped someone, then you're allowed to search them for a weapon-- that's a basic safety issue. The officer (McFadden) is allowed to search Terry because he believes an armed robbery is afoot.
So searching for weapons might well depend on the kind of crime you think is about to be committed. Armed robbery is one thing, but forgery might be another.
Anyway, so you can stop someone and talk to them in order to confirm or dispel your suspicion. This is more efficient and reasonable than requiring that everyone be arrested prior to questioning.
But being searched is offensive. 4A is protecting personal dignity. Scalia: if Thomas Jefferson had been subjected to what Terry was, he'd have shot the cop.
So McFadden was searching for weapons, ostensibly, because he had reasonable suspicion that Terry was armed. What, however, did it mean, when McFadden said that they "didn't look right?"
Practically, we'll never be able to prevent people from making observations on the basis of prejudice. And if we want the police to be able to investigate crimes, we need them to be able to make inquiries.