That's true, but I think there's a solid case for why this type of practice is anti-competitive and unfair.
Let's say I own a restaurant and instead of improving my food or service I go to my competitor's food suppliers and offer them extra money to not sell to any of my competitors. In isolation, there shouldn't be anything wrong with me making an agreement with another private corporation, but the effect of this is that it creates an unfair playing field for my competition.
Amazon's position as both the marketplace and a competitor in that marketplace, creates very similar "unfair" circumstances that benefit them uniquely on that marketplace (for example placing their offerings higher up in search results, or creating special "recommendations" specifically for their offerings, etc) not to mention, the unique access to information that they have into how well their competitors do (which is not inherently unfair, but when combined with all of the other aspects does make a decent case for fitting the "unreasonably constraining competition" section of anti-trust law)
With your restaurant example, you are describing an "exclusive dealing agreement". This usually takes monopolistic conditions to become illegal.
Personally I don't like what Amazon is doing. But I don't think it is illegal under current anti-trust laws. The laws really need to change, but I don't see that happening.
You're right it definitely doesn't fit neatly under any current anti-trust laws.
I think the justification for why something like exclusive dealing agreements in monopolistic conditions is unfair could definitely be applied to why being a competitor in your own marketplace, while leveraging your customer's data, and boosting your own listings could be deemed unfair and lead to monopolistic conditions too.
Those sales were never yours to lose, right?