I think the point is that amp.js will be preloaded anyway, so including it in your package manager means it is downloading content it already has.
. bower install amp
and serving it statically from my server. Why not? Maybe cause Google want to create an audience of people using AMP on their devices.
Because it's wasted bandwidth. AMP can easily intercept the CDNed version and serve a cached copy. If it's part of your bower build, it can't.
I think the point is that amp.js will be preloaded anyway, so including it in your package manager means it is downloading content it already has.