But how? Where is the opt-in? Is including the javascript from ampproject.org considered an opt-in to give gogole a license to republish your copyrighted content?

There is no notice or terms and conditions/license or any kind of other legal agreement that I found on the amp website. [EDIT: actually the code on github is licensed as Apache 2: https://github.com/ampproject/amphtml]

So I don't understand how this works legally. You surely don't automatically sign over your copyright to somebody else by including their javascript library in your page?