google provides the builds to them... they're unlikely to want to push something that breaks things though.
besides, vast majority isn't on any verizon,tmobile/at whatever plan with their nexus7. you see, majority of people with these devices bought them and didn't rent them(like, case of poster, he bought the device from asus. but google branded it and sells it like it's a google device.).
and it's not really a secret why they don't update them all at once: they're beta testing on some regions, like poland or whatever. they fuck up updates all the time, the first updates they roll out inevitably break something.
same goes for apple though, their pushed updates regularly break things.
what is the solution? don't have auto update on. you very rarely get any benefits from updating anyways, like in case of lollipop updates.. there's nothing worthwhile in the update and it has broken several apps for many people! only update when you're doing a phone/tablet reset anyways.