Many users don't even know that's possible

I was talking about the developer. Don't use the platform if you don't like its terms of use.

Sure, of course a platform forbids you from using it to promote its competition. What's weird about it?

Users often have the alternative stores pre-installed. Or you can use your marketing to promote your app through other channels.

It feels like a monopoly due to the network effect, especially with how third party APKs will throw warnings on Android by default.

Well, there's a good reason for the warning.