proactive and accutane are 2 that i know of.
Accutane is a serious medication used when no other treatment has worked for acne. Yes it has potential side effects but it is worth trying for people who would otherwise have scaring from severe acne. Accutane can cause severe birth defects so it should not be used by any women who may get pregnant.
Proactiv is a benzoil peroxide treatment which causes reactions or irritation in some people but has a long history of use fighting acne.
Because they help people... If it was very harmful, obviously they wouldn't sell it. Just because they don't work for you, doesn't mean they are harmful. I've never heard of those being harmful. I know a lot of people who have had wonderful results with proactiv actually.
Actually harmful and dangerous products are sold all the time.