Right now AMP is under heavy development and lots of changes are being made very quickly. That mandatory script loads other resources from the same domain on an as-needed basis, which is a crucial aspect of how AMP is designed to work. I don't know if the v1 release will include a distribution package for self-hosting, but it's not too much of a stretch to believe they've decided to go with a central CDN just to simplify development for the early stages of the project.
At any rate, this is no worse in principle than the Google Analytics scripts everybody includes on every page everywhere, so it's mostly just a bunch of FUD.