Someone smart thought this was a good idea.
"This allows you to use first-party cookies and storage to customize content and simplify analytics integration. Your page appears under your URL instead of the google.com/amp URL."
Creating opacity in security and misdirection in trust is unwise for users. Making the address bar lie is dangerous.
I just don't get how the smart people at Google allow this to happen. The developers of Chrome and AMP can try to rationalize bad behavior but I'm surprised how all their coworkers let it stand. There was once a Google that would have been displeased with this kind of action.