Because your prediction was not what actually happened.
What did mommy and daddy do to little tantrum child?
Among other things, they removed them from the scene. I don't know whether or not they spanked their kid in private, though it wouldn't surprise me if some did -- though it isn't strictly necessary to do so.
It's generally bad practice to do so in public, as it tends to make problems worse rather than better.
You're fighting a losing battle, pal. Real life trumps your pronunciamentos.
It is ABSOLUTELY neccessary that they do so.. and the issue is... not did they throw a tantrum, but did they do it again after being handled properly.