Saw this story the other day. After my wife's 20 years in special education, my own teaching experience with kids who have a variety of traumas, disabilities, and issues, there's a slight point to the "you don't know" comment, which I always try to keep in mind when I see public meltdowns and temper tantrums.
But
The apparent lack of parental supervision (or not admitting the kid is theirs) mostly tosses that out the window in this case. If the parent or guardian knows their kid is going to behave like this, they need to be present and accountable to shut it down ASAP. If the parent of guardian somehow thinks this is acceptable, they need to suffer consequences for their child's actions and likely need some serious intervention, if not removal of the child.
It also saddens me that we've gotten to a point where almost no one was willing to intervene. I don't blame anyone for not intervening, cause odds are very high you'd get slapped with a lawsuit and/or charges, but the fact that there is so much litigation and liability involved in doing the right thing just really feeds this kind of behavior. Wonder how many onlookers have shared/said/stickered "It takes a village to raise a child" at some point in time. Easily said, harder to act on.