General Manager for WP7 Matt Bencke touched on the issue at its core on the Windows Team Blog:
“Tethering, in the past was something that we managed independently of the Mobile Operators (e.g. AT&T, Orange, Verizon). In recent years, the Mobile Operators have now taken ownership of this functionality x-all platforms (e.g. our competitors as well as us). We are continually working with them and our OEM partners to enable this functionality for our devices, but at this time there isn’t an announcement that we’ve made in regards to rolling out this feature. Stay Tuned!”
Wireless carriers have truly went hunting for tethering apps chasing additional profits in the recent months and AT&T has been in the forefront of these efforts, as the carrier sent out warning messages threatening to start charging users additionally even without their consent.
source: Windows Team Blog via WPCentral