I don't think it forces you to include resources from Google. I think the 2 primary reason Google asks you to include a specific javascript script [1] are :
1 - Lazy & Prioritized asset loading logic
2 - Loading assets from cache
Regarding the lazy & prioritized asset loading, the script [2] shouldn't be relying on some Google specific stuff. Since the project is open source, anyone can take a look and get an answer.
Regarding the loading from Google's cache: I haven't dug that much into it, but it's supposedly possible to write your own cache service (instead of relying on the currently free google CDN). Google provided the API and the URL format to so that it should be doable [3]. My assumption is that you can then create your own script, with the cache server URL changed to your own instead of Google's in a simple config file[4].
The question is then: by doing so (and effectively cutting any ties with Google's servers) will your page still be recognized as AMP by Google search engine? It will still reap the actual perf benefits (assuming your CDN does not suck), but if Google's crawler really wants that [1] to be present (and it should not), you wouldn't get the SEO benefits.
[1]
[2]
https://github.com/ampproject/amphtml[3] https://developers.google.com/amp/cache/
[4] https://github.com/ampproject/amphtml/blob/c44a48fbb1dbd0de0...