Again, Terry stops are quite constitutional.
The big lie attached to this whole story by the ignorant and unlettered is that the police have no authority to make Terry stops.
They absolutely do, and only in a left-libertarian's fantasyland are the police supposed to collect paychecks for waiting on thugs to rob or assault someone before they ask them their business.
The appeal to the “ignorant and unlettered” is that there is no difference between a legitimate search and a fishing expedition.
To address your straw man, cops are not required to wait for an assault or robbery to occur as loitering, behaving erratically, and casing people or locations are all reasonable and articulable behaviors to stop someone and ask their business. These actions are not however reasonable suspicion that one is armed or carrying contraband. It appears in NYC that many times the threshold for a stop and frisk was based soley on a cop's opinion, and occasionally his quota. One does not have to be a libertarian to make this distinction but one does have to be a nanny-stater to ignore it.