It seems that sometimes the push is external. A few days ago Canonical announced that they would switch Firefox to snap because that's the only way Mozilla will allow them to redistribute it.
Having a user app like Firefox force autoupdate is not that bad IMHO. Having server infrastructure components update themselves automatically can be disastrous. This caught me by surprise, painfully.
Snap for GUI apps, okay, fine. Snap for system infrastructure, no thank you.
I doubt this is the whole truth, compare https://news.ycombinator.com/item?id=30800957