newyorkguy
Penultimate Amazing
I seriously doubt a judge would give you a warrant to frisk an individual because they have been standing on a corner in a known drug area for the past two hours, yet it is certainly enough to raise suspicions.
Which is why they need to be available. Seeing a person wandering around a neighbourhood and suddenly taking a profound interest in the wall each time you drive by isn't enough to get a warrant, and even if it was, the odds are that by the time you get it, the person will be gone, or have completed whatever it was they set out to do.
I think in the examples you cite there would be no problem with an officer stopping them on the grounds of reasonable suspicion. In fact in New York there is a charge, "Loitering for the Purpose of Using Narcotics."
Here's the statute from New Rochelle NY, a suburban city located outside NY City.
LinkNo person shall loaf or loiter on any street or other public place for the purpose of using illegal drugs...