I think it has always pretty much been the case in this country that laying hands on someone else is considered assault and battery, and when force and intimidation are used like this to steal property, that's robbery.
As to your understanding of use of force, when it is appropriate, and how people are trained, I suspect you are misinformed and lack understanding. (For instance, I'd be curious to hear if there was any struggle before the shooting, or any perception by the officer of an assault on his person. Or did the officer simply decide out of the blue to gun down this young man for fun? It'd be handy if someone had video, for instance, so we could see the body language and movement...)
As to the exact circumstances, the facts matter. Facebook and Twitter aren't facts, or evidence.