I'd imagine a notoriously cash-strapped site like Reddit is enthusiastic that Google is willing to offload traffic originating from Google Search on Mobile to Google's own AMP CDN. This helps incidental visitors still consume Reddit content, while real Reddit users interacting with real website or through a Reddit app are still getting the normal experience. This arrangement only really frays at edge cases, where a logged-in Reddit user uses an external search like Google to look something up.

>I still don't understand why Reddit signed up

Sign up, or face the reduced google referral traffic as AMP-only carousel results push the organic results below the fold.