The issue is not as simple as you make it sound. As you noted, Heinz grabbed a huge market share because “it sold sanitary ketchup… and actively marketed itself as a source of untainted foods….” As you also wrote, Heinz did this *before* the 1906 law was passed. Thus the free market worked — not the government mandate! Of course Heinz would support the law, since it would likely drive some of Heinz’ competitors out of business.
This doesn’t mean no people were harmed by the impure products sold before the 1906 law. It’s what I call “the problem of pain.” Unscrupulous producers can always put bad/dangerous products into the marketplace, and a free market will sooner or later smite them down. The problem is that the harm has already been done, and often no one is held accountable. The FDA can indeed make producers accountable, but only after the fact. I do agree that the threat of fines, prosecution, etc. helps make producers more conscious of causing harm.
Still, the FDA clearly over-regulates. One obvious example is that sick people who are diagnosed with terminal illnesses aren’t allowed to use experimental medicines. Another government overreach is the prohibition of drinking raw milk. If people knowingly want to risk their health, that’s their own business, not our nanny state’s. Hopefully Trump’s intention is solely to abolish this type of thing.