Notably absent in the anti-trust speculation is how the AmazonBasics brand does or doesn't get treated differently from other brands with respect to counterfeits (and other undesirable non-Amazon seller behavior, such as used items as new). AFAIK, Amazon prohibits third-party merchants from selling AmazonBasics, which would mean that the inventory-mingling problem, which has been discussed on other threads, wouldn't…
Commingled inventory is an option for FBA sellers. You can disable it easily from your FBA account; I think at this point the "problem" is only for beginning sellers who aren't aware that it's on by default
An unscrupulous seller (of, for example, couterfeits) would benefit significantly and so would not disable the option.
That Amazon themselves don't disable this option (for "shipped and sold by Amazon.com"), except, effectively, for their own brand, is the suspect anti-competitive practice.