It appear that sending encrypted content is a bet on whether the user pays for the content or not. I'd wager that for most page loads people won't go on to pay. So if page-loads become larger on average due to the encrypted content then the real benefit here is that the original content providers will be hit less (since amp can fetch the content all at once), but perhaps more importantly, google can index this content.
edit: So in total, users' bandwidth won't be saved - google is just serving itself, but packaging it as a user benefit.
edit: So in total, users' bandwidth won't be saved - google is just serving itself, but packaging it as a user benefit.