[ledneb]

Nabbed from the comments of the post, this is how you disable the "feature" without being rooted:

http://vinhboy.com/blog/2013/12/27/how-to-tether-the-nexus-5...

... still though, I'm interested: what actually is this second device which things get routed through? How can the carrier tell which device your phone is using? Even then, how do they tell which one is the 'normal data' vs the 'tethered data' device?

[kzahel]

I spent a day trying to overcome the tethering restriction on my T-Mobile unlimited data prepaid Moto-G. I tried changing some tether_dun_required setting (mentioned in parent), which required me to root my phone. That didn't have any effect!

I ended up running a SOCKS5 proxy on my android phone (the app is called Proxoid, and is unfortunately rather buggy). That at least works, though with the app being rather prone to crashes and hangs, it is not the ideal solution.

[blueskin_]

Different devices go to different APN gateways as I understood it.

[thefreeman]

the different devices are assigned different IP addresses