This type of marketing is certainly not exclusive to equine supplements. Look at the cosmetics industry!
However, since this is an entirely unregulated industry, you can put floor sweepings in a jar and if people think it works . . .
What bothers me is advertising/marketing that plays on people's desire to do something good.
I had my wakeup call about 7 years ago when I had a equine nutritionist sit down with me at the barn, look at my horse and evaluate what I was feeding. And what I didn't need to feed -- I immediately saved a bunch of $$