Misrepresentation is commonplace, even outright fraud is commonplace. Serious fraud is a multi-billion dollar industry. "Light fraud" and gray area misrepresentation is at least hundreds of billions of dollars a year.
The "free market" doesn't make fraud or misrepresentation illegal, the laws and regulations of the dreaded government do.
The worst the free market can do to a fraudster is to stop dealing with her as knowledge of her frauds becomes widely known. That, of course, doesn't work very well to limit fraud.
Well... we're generally used to "adulterants" referring to inactive or even potentially (purely) harmful ingredients being added to things to make them cheaper. However, note that "adulterating" your sexual potency supplement with Viagra means that it will, you know... work. Same with putting steroids in your workout supplement... it makes it work, better than the competition that doesn't do it.
There are all kinds of other problems with that, of course. I'm not saying it's a good idea. But in terms of "why would people buy these?", well, I'd suggest part of the reason is that people want to lose weight, be sexually potent, or get swole, and these "adulterated" supplements actually work. Just at a cost you may not have realized you were paying, or intended to pay.
That's obviously false.