What the hell, I usually don't get into to this stuff but I am in a mood

#1 The kid is a fucking idiot, YES it is your right, YES it is Legal to do so, but it is this kind of whistle dick that makes most "rational" gun owners look stupid.
#2 I am sure the Police know the law, did he try to push it a bit too far, yes. Make contact and determine intent. There is the other video out there where LE did it right, the first thing out of the LEO's mouth was something like "Thank you for exercising your 2A Rights. Just a couple questions and you will be on your way." That is all it takes.
#3 Somebody WILL call 911, fuck they call 911 for a drunk passed out on Colfax, there is drunk passed out on Colfax every other block. LE has a DUTY TO RESPOND and MAKE CONTACT.

Disclosure - Cops have saved my ass more than once, I respect them and the job they do.