If your grocery store "willfully acquired or maintained monopoly power by engaging in anticompetitive conduct".. then you'd be actively and purposefully affecting the ability for anyone to "try to build an alternative to compete with [it]".

They aren't asking Google to use a specific price, what they are asking is for them to stop offering special custom-made deals under the table for some of the partners with the intent of preventing competition. Nobody is stopping Google from offering the same fees to everyone indiscriminately... the issue is when they pick and choose with the purpose of minimizing/discouraging competition. Particularly when they are already the biggest one in their market by a wide margin, so they have a higher power/responsibility than a Mom'n'Pop store and can more easily manipulate the market.

You really need to read the article, and specifically the linked article within that details the court proceedings. Anticompetitive behavior is illegal, and Google did lots of it, and did so blatantly, and deleted evidence of doing so.

The 30% they charge isn't the issue. The issue is the anticompetitive actions they took to keep themselves from ever having to charge less than 30%.

If that were all this was, sure. In your analogy though, Google owns 95% of the grocery stores and has deals with 90% of the food vendors that if they allow you to stock their brands they lose access to sell in the Google grocery store. That practice is anticompetitive, because it functionally prevents you from opening your own store to compete.

Problem is that it not really is "just a store". By using the google store you get access to the google play APIs, which are upgraded separately from the device OS - which is sensible from a security perspective, but they also were created by google specifically for regaining control over what goes on on Android devices.

A lot of applications are needlessly tied to play APIs - either because that way is a bit easier, or just because google is good at marketing them, and the developer didn't think twice about it. Some relatively basic APIs are part of google play - for example maps, which needlessly is tied to google maps. Unlike Android itself the play APIs are not opensource.

Yandex tried about a decade ago to re-implement the play APIs to keep such applications working without the play store, by utilizing other services providing the same functionality, and tried to get other companies to join them. I've visited the Yandex office in Saint Petersburg a few times to discuss that back then (just checked, most of that seems to have been 2014 - that year Yandex was sponsoring my Russian visa). The effort failed for various reasons, unfortunately - the big one being that doing this required reverse engineering API changes on every play update google was pushing to stay compatible. There's the microG project around now, but it seems to be less ambitious than what Yandex was trying to do back then.

My point is, as long as at least the API for play services isn't maintained in a way that allows full open source reimplementations - or better, google releases parts as open source where we can plug different backends in - "use a different store" is not really a possible solution for many.