What grocery store let random people stock their shelves?

There is no oversight of supplements (at least in the US). You just have to trust the brand, because legally there's nothing stopping a supplement manufacturer from using entirely different ingredients from what is claimed.

Yes, but at least you can trust the brand or not at most brick and mortar stores.

On Amazon, you can buy your trusted brand and not actually be getting that brand.

Granted, this happens at brick and mortar on occasion, but they generally have a relationship with most of their suppliers and it would be unwise to allow counterfeits through.