It's not just anti-trust, it's also trade secret laws. A customer of AWS has a reasonable expectation that the information it keeps on AWS's VMs are confidential.

Is that comparable to the owner of a mall watching who goes in and out of which shops to decide what stores to add to the mall?

Isn't it more like the mall owner opening a clone of your store right next to yours while charging themselves no rent in order to gain an advantage, all the while promoting their own store they opened to steal your business on the ad boards situated around the mall?

Is that illegal?

> A customer of AWS has a reasonable expectation that the information it keeps on AWS's VMs are confidential.

This is where End User Agreements may be worth checking. There may be a specific clause AWS customers agree to.

Is AWS a monopoly?

One can violate antitrust laws without being a monopoly. Certain parts of the law (regarding collusion on price setting, for example) can be broken by very small businesses.

See this helpful FTC page: https://www.ftc.gov/tips-advice/competition-guidance/guide-a...

This is true, but those are all cases in which putative competitors collude to essentially form a cartel. Which is a distinct category of antitrust offenses from anti-competitive behavior.

As others note you don't need to be a monopoly to violate anti-trust laws. However, as it relates to being defined as a monopoly this ability to leverage your market position to stifle competition is the exact type of behavior that would support a finding of monopoly...most non-monopolies can't leverage their market position to unfairly compete

Isn't the important factor whether it's trying to become one? Windows and IE were never the only possible options.

A de facto monopoly doesn't mean that other options don't exist. Microsoft had a monopoly on the PC operating system market, despite other options existing.