I think the argument is around anti-competitive behavior based on them controlling both the platform (Amazon Marketplace) and their own store (Amazon Basics/Whole Foods). This Verge article [1] cites the Diapers.com incident, where they basically cut prices until Diapers.com couldn't compete and had no choice but to be acquired.

There are also the ongoing investigations in Germany around anti-competitive behaviors. [2]

I'm not making a judgement on validity, just pointing to what people are likely referencing.

[1] https://www.theverge.com/2018/9/5/17805162/monopoly-antitrus...

[2] https://www.reuters.com/article/us-amazon-com-germany/amazon...