AFAIK, it doesn't. 4.4 contains a setting that allows a provider to specify a different APN for tethering use, which I can imagine is a legitimate feature. You can also turn it off (on IPv4 at least)
The same Nexus 4 + Android KitKat 4.4 + AT&T SIM Card does not detect unauthorized tethering.
Surprisingly this "feature" only appears when using T-Mobile SIM Card.
It is not the case with Android Jelly Bean 4.3