What is the deal with salon products sold in grocery stores?

I’ve always heard warnings from hairdressers not to purchase salon shampoos and products from grocery stores (or Walmart), and I’ve always wondered why. I’ve had multiple stylists tell me that they are not genuine products, but I don’t see how that is possible. I just don’t see major retailers selling shelves upon shelves of counterfeit hair products.

So is the issue that salon products are not guaranteed unless they are sold in a salon? Or are they really counterfeit?
And if it is that they are not guaranteed unless sold in a salon – why is that?