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?