The issue here isn't people adding sesame and not mentioning it. The problem is that if there were something else with sesame using the same machinery, anywhere

The issue here isn't people adding sesame and not mentioning it. The problem is that if there were something else with sesame using the same machinery, anywhere along the line, you can't simply say "this item might have traces of sesame" (you're still liable, per the FDA) and you can't say "this item has sesame" (because it doesn't; it just has possible sesame cross-contamination). You have to either have fully eliminated the possibility of cross-contamination or make sesame an explicit ingredient. One is free; one is expensive.