I was reading about the laws of bitul, though I've never learnt about them properly in any detail.
One example given in the book (An Artscroll book on kashrut) was that you can assume that a grocery store uses a knife only for watermelons... and that even if it's possible that the knife is not kosher, it would become so after a few cuts and the first few fruits cut would be nullified among the rest.
Based on the principles in this reasoning, why are some people extra stringent to avoid cholov stam milk and other products that are given kosher status by kosher lists but don't have a hechsher? It totally makes sense to me that you would want to avoid any possibility of contamination, but since it can apparently be nullified with a high enough quantity of the kosher ingredient (as would tend to be present), and since you can assume so much... what is the difference between the possibly non-kosher knife used to cut fruit and the machinery used to process something else? If it's heretical to say you can't eat actually non-kosher meat mixed in by accident with kosher meat in particular situations, then how is the extra caution necessary?
If there a factor of bitul only counting 'after the fact' so that you have to be cautious still with processes, why doesn't this apply in the case of the fruit from a grocery store? Is it because there more likelihood in the 'assumption' in that case? And if so, why the problem with cholov stam milk in the eyes of some people?