He didn't say anybody (other than United) was responsible.
> Not making sure an unaccompanied minor gets to her next flight is not "lack of human decency".
I'd say not helping a fellow human in distress, especially a vulnerable one (such as a child) in a situation where you can help at next to no cost, definitely qualifies for "lack of human decency".
> "I want to know who the the passengers were sitting next to this child were that didn't help her with her transfer."
And, no, you don't know she was in obvious distress, nor do you know the passengers next to her were aware of her problem as they were disembarking.
Not taking the time to help a child to the next gate is normal, any rational actor would expect the airline to take care of it; the fact that they did not does not mean the other passengers lack human decency, what the hell.