[mcbits]

They have poor naming. There is an Advertising API, and then there's a Product Advertising API which is often called the Product API.

The Advertising API is for advertisers running ads with Amazon Advertising. Apparently they don't allow certain competitors (shopping sites, comparison sites, ad networks) to use it: https://advertising.amazon.com/en-us/resources/ad-policy/api

The Product API is for affiliates to fetch product/price information in a reliable way instead of scraping the main site for it. I don't think they've ever prohibited price comparison sites. The last time I really looked into it was several years ago, and they weren't restricted at that point either.