You're welcome to search for mentions of sources and app stores, and to browse the references.
Again, the paper is not in contradiction, you misread it. Not to mention, as explained in the original comment you were responding to, F-Droid weakens the security model for various other reasons.
It's obvious you're convinced you're right, but since you're unable to produce evidence it seems we are at an impasse. I'm done here.
You have been provided with evidences, but you've been arguing in bad faith since your first comment by misinterpreting the paper. The security model also expects you to download apps from trusted sources because the signature verification is only enforced for app updates, that's a trust-on-first-use model. That notion exists within Android, and you have been given examples.
> In order to ensure that it is the app developer and not another party that is consenting, applications are signed by the developer.
Back to the original question; it is clearly explained why.
In return, you suggest that the documentation is out there, but you give no specific pages that I can read. My attempts to find them on my own or through your blog post have been fruitless. Given that the idea that you're putting forward runs directly contrary to the paper that Google published and is contradicted by the behavior of Google's web browser, the burden of proof is now on you. I need your source. Please provide a link to where you arrived at your idea.