AMP can impose constraints because it's a corporate effort to re-frame content in a way that provides centralized ad serving, tracking, and the like. It gives the content publishers the same tools (ads, tracking) they have to provide themselves with on the open web, while simultaneously benefiting those like Google who will put their own viewport and context around that content. It's not that much different from when an individual wants to publish on Medium or Tumblr, so the platform provides the author with a box to put text in, a dashboard, view tracking, and stuff the author wants, while they get to host interesting content with which to attract an audience for ads. It's a win-win, symbiotic relationship, while on the self-hosted web, every publisher is on their own.
This isn't about HTML vs AMP at all. It's about business models, which are given in AMP, but left as an exercise for the publisher on the open web.