Amazon claims to be a neutral sales platform to third-party sellers. Originally, if you wanted to sell there, they’d list you, process the payment, take a cut of your sale price, raise the orders to you and then you would handle order fulfilment yourself.
Then Amazon started to offer its own fulfilment as an option to business. That meant that you sent your stock to an Amazon warehouse, and they did the shipment (and took another cut of the selling price for the privilege).
That’s perfectly legitimate: a business can choose to have Amazon as their fulfilment service or not. Where it becomes a problem is that Amazon won’t fulfil orders that are not generated on Amazon’s sales site, so you have to maintain a separate fulfilment operation if you operate your own website or sell through other channels.
You’d think that this restriction of “only Amazon orders fulfilled” would make Amazon fulfilment unattractive to sellers with existing sales channels in place, and obviously so did Amazon, so that’s why it stepped over the line of what’s legal, and started limiting its promotional services, search placement and advertising campaigns on the on-line sales platform only to sellers who were also paying Amazon to fulfil the orders. That’s a classic case of anti-trust activity: using leverage in one market to remove competition in another.