I am curious if there actually are any laws relevant to this situation. As far as I can tell, this is exactly like Walmart or BigBoxCo pushing their own brands. It's not illegal and, generally, it's not even a bad thing for the consumer (in the short term).
It really feels like most of the people taking issue with this are doing so because of their personal sentiments rather than legality.
At one point when coffee wasn’t a big section, the category captain of coffee actually helped bring in competitors to increase the overall size of the shelf space and thus attract more passing customers.
If this was you or I, there wouldn't be a problem. If you are the only / pretty much the only store used then it (to me) would clearly be a problem for consumers. You'd be using your monopoly in one area (marketplace) to benefit you elsewhere (selling USB cables) rather than competing on a level playing field.
In between it becomes trickier. At what point are they too big? How dominant would they need to be?
I think there are differences to physical stores partly due to scale. If a shelf has 10 different variations of basically the same thing that's the equivalent of all products being on the front of the search results. If there are hundreds, then the ranking becomes vital.
Downvotes for bad information, not snark
AFAIK US antitrust law does not have any concept of “marketplaces.”