The AMP spec REQUIRES you include a Google controlled JavaScript URL with the AMP runtime. So technically the whole signing bit is a little moot, given that the JS could do whatever it wanted.
The same could be said of any CDN hosted javascript library. For example: jquery. There is an open intent to implement support for publishers self-hosting the AMP library as well.
For most JS served by CDN, you can (and should) use Subresource Integrity to verify the content. At least the last time I was involved in an AMP project, Google considered AMP to be an "evergreen" project and did not allow publishers to lock in to a specific version.
"Registrant Organization: Google LLC
Registrant State/Province: CA
Registrant Country: US
Admin Organization: Google LLC"
Note that jQuery, as mentioned in some GP comment has no such requirement. Google AMP is quite unique in this regard. This is NOT some general CDN type issue. Also...agreed, WTF is "open intent"?