We tried letting corporations police themselves early on, and the resulting chaos and snake oil salesmen prompted the implementation of the FDA to begin with. Too many people are too easily taken in by unproven claims, and the temptation to cut corners is too strong. Who’s to say that companies might not start including stronger substances, or stop quality checking altogether. Without the incentive to follow the rules, too many people would be injured or sickened by bad products. I would suggest that regulating the content of comestibles for dangerous substances or proper quality would fall under protecting the general welfare, something the government is authorized to do.
(http://www.mises.org/story/1805 , http://www.mises.org/misesreview_detail.aspx?control=272 - a bit too anti-government, but shows some history of the FDA and conversation on current approved drugs.)