I just dont really see the point to this hypothetical when it is obvious thanking is proper manners. Judging people on one action is kind of misguided, if people picked one instance of my parenting/life there are certainly some bad ones to stumble on. Who knows if those parents do approve of their children swarming? That they arent caught off guard and embarrassed?

Yes offering and thanking is the "correct" etiquette, yes some people fall short but who hasnt in some way at some time? Just because somebody doesnt verbalise thankyou doesnt mean they arent and just because they do doesnt mean they are. Doesnt hurt to give benefit of the doubt and a bit of slack.