When Google yanked tethering apps from the Android Market, anger broiled in customers and developers alike. Google's response is simple, but has far-reaching implications: tethering apps are back, just not for T-Mobile USA customers.
What this means is that means that listed apps can be defined as carrier specific, by Google. A tethering app that violates T-Mobile's ToS will be either invisible or inaccessible to US users, while available to someone on a tethering-friendly network elsewhere. This has the effect of creating carrier-specific App Markets, and an uneven Android experience from company to company.
Of course, you can still install whatever apps you want if you're willing to get your hands a little dirty, but removing a category of app from the Marketplace will make it effectively off limits for most. [Android Community]