Actually, the reason the FDA doesn't approve them is because their hands are tied - since 1994 thanks (or no thanks) to the Dietary Supplement
Health and Education Act (DSHEA) which you can read about here.

No matter what people's opinions are about the FDA - overall IMO I think they do a good job - I trust THEM a lot more than the vast majority of the supplement pushers, who basically can say whatever they want about their largely spurious products, including posting a lot of faux studies on their website, calling a pill a "Hoodia product" when there is little or no Hoodia actually in them (no big deal actually, since the stuff doesn't work unless it is the actual fresh plant - it always kills me how the Hoodia pushers tout "As Seen on 60 Minutes" in so many of their ads and when you read the transcript on the CBS site - it clearly states only the raw plant has the appetite-supressing qualities :rolleyes: ).

Back to the topic of 'cleansing' - we have a bunch of threads here on the topic but basically, your own system does a very adequate job of 'cleaning your system' most of the time, without having to use these products (and just because they're 'all-natural' doesn't mean they're harmless or kind to your body).