You don't need to be a perfect monopoly for antitrust laws to apply to you in the US. You just need to be in a "dominant position" in the relevant market. Even

You don't need to be a perfect monopoly for antitrust laws to apply to you in the US. You just need to be in a "dominant position" in the relevant market. Even if Google allows side-loading, so long as 95% of users go through the app store they need to take care not to abuse that position.