I can't imagine any of my friends being upset with the fact that their children should behave in a restaurant. Either the moms in the story have no control over their kids, or there is another issue going on that didn't make it into the story.
I think it is the former - moms with no control. I can recall grocery shopping when my two children were still riding in the grocery cart. We'd hear loud, boisterous, and often rude children in another aisle and look at each other knowingly. Well behaved children are made, and a blessing.
I'd guess the owner stood his ground, resulting in the 'angry' boycott leader, who probably cursed and yelled at him (and in front of her child) for 'offending' her and her child, then went on her mission of destruction.
She seems a perfect example of an 'adult child'. They're raising more of the same. Her kid(s) will be even worse, led by example. Few people stand up to this type, so they think they're in the right (the logic: nobody's disagreeing with me, so everyone must agree with me). More American rot, from lack of caring cultivation.
The mothers themselves like to yak at the top of their voices even in good restaurants.