The amount of discipline a parent (limited by the time and willpower a parent can input into the process) can impose is finite. Once we accept this hypothesis, we can see how usual norms are imposed by parental discipline plus non-parental discipline. A parent going against a norm will have to use an extraordinary amount of time and willpower to impose his unconventional norm, as he will lack the enforcement of the non-parent actors. Eating sugary cereal is a societal norm in today's developed countries. Therefore, a parent wishing to diverge from this norm would have to expend an extraordinary effort.

He's also going against the professionals who already succeeded in imposing the norm and simply have to maintain it today.