Blacks have an aversion to authority and naturally test it at all times. In a black community authority/hierarchy is gained by aggression and posturing so it is natural for them to always resist any demands of them with that aggression and posturing or else they will remain at the bottom of their own hierarchy at all times.
Something as simple as just informing them of a business's rules or that they have to get in a line can trigger them not because they don't understand the rules but because they view this as a dominance ritual and they have no intention of losing that game.
If you really pick apart what they are all saying about Karmelo you can find that it is essentially "You cant tell us what to do and if you do we can act however we want". If you study other video interactions you will very often hear a line like "You can't tell me what to do". It is always based around them feeling that any request of them is a flex of authority over them and submission is out of the question.
This case means something to them because it challenges this philosophy. You will notice they will say something like this above quote "black children have boundaries" what they are saying here is that blacks must not ever feel a sense of domination in the slightest or they will act. She brazenly states that Austin's father failed in educating him on this, in a way she is correct. Never relax, always stay hyper vigilant.
The lesson in all this and instances like Travon Martin isn't that they are confused on what self defense means it is that they have a different social system all together. These instances make them collectively feel that same sense of domination. It bothers them deeply.
They don't want fairness, they want to win.