IMO it was a set-up. A sting. Looks to me like it was very carefully planned, to maximize the "piss him off" potential, and, to start the ball rolling by giving an answer indicating that the kid was high -- but doing it in a very Clintonian "plausible deniability" way (high/hi).
So, the cop screwed up, but it was due to some heavy duty "bear-baiting."
This kid ain't no poster boy for the Bill of Rights. (And I'd really like to know who he's sock-puppeting for. ACLU? Soros? Something stinks really fishy about the whole thing.)
Obviously the police set themselves up for it too, if for no other reason than being there. All this "papers, please?" crap is an open invitation to all sorts of abuse, from all directions. It's bad, period. It does NOT belong in a free country.
Were you people actually serious about this? The first time I read this, I thought it was a tongue-in-cheek joke, but the more I see people bring this up, I think they are actually serious.
The policeman said hello. See, in normal circles, when one person greets you, it's typical to greet them back. Do you find yourself wondering why people ignore you or give you strange looks? Why they think you are anti-social? Let me give you some tips on what otherwise might be an awkward meeting. I'll be Person 1, and you can follow along and pretend to be Person 2.
Person 1: Hi, how ya doing?
Person 2: Hey.
--Or--
Person 1: Hey, what's up?
Person 2: Hi.
--Or--
Person 1: Hi there.
Person 2: Hello. How are you today?
See, if you follow these simple rules, you too can be a functioning member of society. Give it a shot.