I'm faced with the same problem and yet I'm still trying to understand what's going on..
With the Configuration Utility you may set the APN's for the mobile Network, but these already do running well by default. It's the APN's for Tethering and MMS, wich doesn't do, plus no carrier-logo as well.
And it's not that the Tethering-option gets disabled by that bundle. Instead, the service keeps steady running, looking for a chance to switch on and drains power for nothing.
The most annoying thing is, that congstar generally allows Tethering. On any setable device it'll work. Without a specific carrier bundle, iOS wouldn't hide the manual settings.
So obviously, congstar is just not able to change anything about their bundles, if the big T-Mob-Mama don't want them to..
A workaround, to make the Tethering-option swichable with a constar SIM, is to put in a current T-Mobile microSIM that allows the service. After that, you can at least turn the service clearly off, even with your congstar card inserted. When you try to turn it on, it will just generate a wlan that other devices can't connect to.
In the end, it clearly seems like T-Mobile decides about how you spend your paid bits and bytes with congstar WHEN you're using an iPhone.
