It’s no secret that carriers like Verizon and AT&T do not like tethering. They only like tethering when you pay for more expensive data plans that allow it, but if you don’t have such a plan, you’re not allowed, by the terms of service, to use tethering applications. […] This is now coming to an end, as carriers are working together with Google to block certain devices from installing these tethering devices from the Android Market.
http://www.osnews.com/story/24692/US_Carriers_Work_with_Google_to_Block_Tethering