R' Mordechai Kamenetzky answers that the difference is that the story of the Ark is a narrative, which is when people should be careful to tell it over in a nice way, refraining from Lashon Hara (slander) or negativity of any sort. In our Parsha, however, the Torah describes the nitty-gritty laws of what one may eat. In our case, it’s important to give a resounding "TAMEI!" when discussing these matters, as the consequences are much graver. It should be the same when dealing with children and others around us who may not know better. We speak softly in order to get them to understand history, reasons and customs of Judaism. However, as the metaphor of food may hint at, if they are in imminent danger of internalizing negative influences, it's time to fearlessly admonish them! When dealing with clear right and wrong, the Torah tells us that sometimes it’s necessary to boldly speak where no one has spoken before!