Nexus 7 3G bought outright entirely separately to any plan or network carrier. Tethering is entirely disabled and many forums confirm this.
And while I theoretically could I am not going to install a "cracked" version of Android from a l33td00d7 user on a php forum just to get a fundamentally basic and useful networking feature which is present in other installs of Android and which there was no reason to turn off. The whole point of buying a Nexus device was to get away from the crap modifications (like the constant nagging, with a loud beep, to update the "Facebook integration" stuff from my phone's manufacturer which can't be turned off) and lack of basic updates on non-pure Android devices. I wanted a pure Android experience, one of my strong desires was to use it for tethering, Google promised a pure experience and it was a deception (can't remember if it was actually stated somewhere in the fine print, but I certainly read loads about the specifications before purchase and didn't see any mention of it).
It's a disgrace and, along with a series of other pointless Apple-like restrictions worming their way into Android, it confirms Google is fully prepared to cripple things in the name of profits. And it's not even their own profits in this case, it was likely done at the behest of carriers and hugely damages what Nexus was supposed to be about and their brand, so it's even crazier.
So while Canonical might try to put Amazon integration into the Unity bar, they know that you have to be able to turn it off or they will lose their users. The people at Google on the other hand know they can get away with it, although they might lose a few power users (post hoc, after taking their cash and disappointing them) they'd get a few more tie-ins with carriers, so they did it. What bugs me is that lots of people here work for Google and like to think of themselves as hackers fighting the good fight at the "do no evil" company. No, hackers don't ship devices with basic networking features crippled whilst pretending it's a restriction free device.
I can't wait to wipe Android off my Nexus 7 once there is a robust Ubuntu release for it, and my next phone almost certainly won't be Android.