I'm not buying either of these reasons. This is Google we're talking about. Google has massive server farms that handle billions (probably more) of queries every day. They're serving up cached images. They're running their own app engine (similar to AWS). If anyone has server infrastructure that can handle over-the-air updates to Nexus devices (which aren't nearly as popular as Samsung Galaxy S# ones, let's face it), it's Google.
And the cellular networks should have nothing to do with it. If I'm remembering correctly, Google won't even let me download the update unless I'm connected to wireless (and if it that's not the case, maybe it should be if they're worried about the carriers).
In any case, we're talking about 20 MB or 60 MB updates. People are now paying for data (unless you're grandparented in on an unlimited plan), so if you pay for that 60 MB of data, the carrier should deliver, and they shouldn't care if it's an OTA or Netflix streaming—data is data.
The most plausible explanation is that Google isn't sure if updates are ready when they're "ready" and so wants to do a slow rollout. Even then, I think they should allow users to opt in (sure, don't pull the update automatically for everyone, but if a user manually selects to check for updates, it should check and pull the latest update).
tl;dr there is no valid reason for Google to withhold released updates from users checking for the updates manually.