I've checked & CyanogenMod 11M5 on my Nexus 4 doesn't appear to be doing this - if I turn on tethering over bluetooth there's no extra route in the routing table.
However, my current carrier allows me to tether the data I pay for, so perhaps it only does it in response to some kind of carrier request to split the data into tethered / non-tethered classes?
However, my current carrier allows me to tether the data I pay for, so perhaps it only does it in response to some kind of carrier request to split the data into tethered / non-tethered classes?