This isnt true. You're right in the sense that bits are not transferred from publisher's servers, but AMP page can report metrics as instrumented by the publish

This isnt true. You're right in the sense that bits are not transferred from publisher's servers, but AMP page can report metrics as instrumented by the publisher, ads get attributed to the publisher and links back to the publisher. It is a cache, and behaves like a cache. You're confusing this with an illegal upload of a video to a different site. The other site is 'stealing' the traffic, and the original creator is losing $$, ad impressions, attribution, exposure etc. None of that is happening with AMP Cache.