Hadley points to the serious security concerns with AMP:
Fundamentally, we think that it’s crucial to the web ecosystem for you to understand where content comes from and for the browser to protect you from harm. We are seriously concerned about publication strategies that undermine them.
Andrew goes into more detail:
The anchor element is designed to allow one website to refer visitors to content on another website, whilst retaining all the features of the web platform. We encourage distribution platforms to use this mechanism where appropriate. We encourage the loading of pages from original source origins, rather than re-hosted, non-canonical locations.
That last sentence there? That’s what I’m talking about!