I'm not sure if the beat down they give you is a "right", as you appear to be saying it is. The problem is that in certain altercations, it's a he said, she said kinda deal. If there are no other witnesses, and one of the two parties involved in the altercation dies, then it's really just the word of the survivor, and whatever physical evidence the police can gather to try to either corroborate, or dispute his/her story. If the survivor has zero wounds, and the person who was shot also has obvious signs of physical trauma before the shot, then it's going to be hard to believe if there's a claim by the survivor that the other guy was assaulting him, and that's why he shot him.
I think it's easy to confuse retaliation and self-defense. In your bar scenario, where you piss in someone's beer: the guy who got his beer pissed in is not defending himself, he's retaliating, or trying to get even with the guy who pissed in his beer. There is no law (to my knowledge) that condones retaliation/getting even.
Here's another scenario for ya: Woman has suffered continuous physical abuse at the hand of her husband over a lengthy period of time. One night, the husband is verbally insulting the wife, and getting in her face, etc. Frightened, she slaps him across the face, and one of her rings leaves a small cut on his face. Ok, so at this point, what is the husband permitted to do? He weighs 220 lbs, she weighs 105 lbs. She started "the fight", so to speak. To what extent is he allowed to retaliate? Is it reasonable to believe that her slap to his face, and the small cut he received, has given him a reason to fear for his life? So, he starts wailing on her, grabbing her by the hair, throwing her against the wall; punching her in the face, knocking her to the ground. She runs to the bedroom, and he chases her in there. She's in the corner, between the bed and the wall, and as he begins to lunge towards her she pulls a gun out from under the bed and shoots him.
Now, she "started the fight", so she was the initial aggressor, and there is physical evidence (the cut on his face) to confirm this, and she admits it as well. I believe, in some states, without the SYG law, that she could get charged with the murder. I think that's the point of the SYG law, to protect people in those types of scenarios. The law isn't perfect, and it is abusable, but without it, people like her potentially spend years behind bars.