[tokuzumi]

There's probably some type of trigger in the packets sent from the phone by WModem that would indicate the phone is being tethered to a PC. The packets sent using WMWiFiRouter and WiFiRouter are most likely identical to the packets sent when using your web browser on the phone, or any other application on the phone that requires web access.

Edit: And the WModem program probably routes all traffic through one of their servers, just like Opera Mobile/Mini and Skyfire do.