I guess I don't understand. My impression is that it is up to the individual, under "stand your ground" laws, to determine whether he or she is in danger and respond accordingly.
That's not how it works. "Stand your ground" laws remove the duty to retreat, but that's all they do -- all other elements of self-defense law still apply. Broadly speaking, three elements that must be present to exonerate an individual in a self-defense killing are: 1) the individual must believe that he is in danger of death or serious injury; 2) that belief must be held for reasons that a "reasonable person" would consider to be valid; and 3) the individual must use no more force than is reasonably necessary to defend against the danger. You can't just say you felt threatened and start blazing away.
If one sees someone aiming at them through a gun's scope while one is sitting in a bar
In this case, the scope was not attached to a gun. If he had actually been pointing a gun at the bar instead of just looking at the bar thru a scope, he would have been committing a crime and would most likely have been arrested for it.
surely that's more of an imminent threat than a black guy approaching one on the street, no?
It would be, if that was what had happened, but it isn't.
It's just odd to me that is situations where police may not be cleared to act, individuals can be.
I'm not sure what you mean by this.
What am I missing?
It's just that, as in so many other areas regarding legal matters, the law is a lot more complicated than most people think it is.