Is their a legal distinction that establishes Amazon as a platform first, and a seller second?
The issue in this case would be that Amazon is manipulating the markets involved by effectively being the market. If FooCorp was a minor player in the market, didn't buy up competing marketplaces, and was clear about it's brand as a seller of Foo products that would be fine (think Costco and Kirkland Signature branded products). The issue with Amazon is that it is half of all eCommerce, buys up competing marketplaces to shutter them, it is hellishly difficult to determine what is an Amazon sold product and a marketplace sold product on their site, and their brand is as a marketplace.