While agree people are focusing on the wrong part of Amazon for arguments of anti-trust regulation you are also ignoring a huge part of why it IS a monopoly. Think of two things: one is price discrimination, being able to change the prices of products and thus affect which ones consumers buy (a holy grail for online retailing but easily a slippery slope). The other one is its in house brand that many times displace other products for the top of the search results slots. Think about what happens when you combine those two, displace non in house products further down the list and charge more for them.
Anti trust law doesn’t deal with size (Apple is a massive company and I doubt they’ll be slapped by anti-trust regulation soon) but with how your power as a large player is abused. It doesn’t matter that Amazon controls over 90% of the online retail market, if they use that power to promote their own products by pushing competitors to unfavorable positions and cutting prices because they have the power to control ALL prices then yeah they are a monopoly abusing power and subject to US anti-trust law.
Yes Amazon is a monopoly, no it is not abusing its power (as far as we are concerned) and thus not subject to much of Sherman act regulation. There is a huge differentiation between the two.