In practice there is no technical problems for Microsoft adding USB tethering in Windows Phone. It's probably already there but disabled or not deployed. USB drivers already exists for this particular usage. The likely reason for Microsoft for not adding USB tethering is because it is a gray zone when it comes to tethering and different operators have different interpretations. Windows Phone supports Wifi and Bluetooth tethering which supports more than one client, then there is not the gray zone as the phone will be regarded as a "wifi hotspot" for sure.
So if I'm not completely wrong, this issue is stuck at the legal department at Microsoft and as Microsoft is a US company, operator regulations are more heavily guarded there.
So if I'm not completely wrong, this issue is stuck at the legal department at Microsoft and as Microsoft is a US company, operator regulations are more heavily guarded there.