Apparently, T-Mobile accidentally offered free tethering all this time. Though they had a service for tethering for $15 per month, people didn’t need to buy it. They could just tether on Android, and T-Mobile couldn’t charge them. I know AT&T uses a special system app to keep you from tethering, which, when frozen/deleted with root, unlocks free tethering. Well this is probably what T-Mo is adding to their Ice Cream Sandwich updates.
After this ICS update, unrooted HTC Sensation 4G users will not be able to tether for free anymore, and will have to use the hotspot plan. While many people won’t like this, it’s only fair for T-Mobile. Sure, we pay for our data and should be able to use it any way we want. But T-Mobile was the only one not to charge for it, and them doing so now is only competition. If you want to keep tethering for free, you can always root. But if you’re a stock Sensation user, you’ll have to either pay for a tethering plan or stop tethering. This will also probably be coming to every phone that’s getting the ICS update too. So brace yourselves.
Source: Android Police