Get 'em hooked on something 'free', then monetize it. Rinse and repeat. Just like they are building a browser that only blocks the 'annoying ads', now they are designing their email to let advertising through. So much for 'don't be evil'.
AMP is afaik an open web format, what's the "get 'em hooked" part here? Is it just a defensive play against Facebook's Instant Articles?