T-Mobile hasn't been actively monitoring users who use their phone to tether for free, but thanks to Android 4.0 ICS upgrades coming, they will be. The carrier has had their hotspot service in place for over a year now, yet because of limitations with previous versions of Android, they haven't been monitoring that kind of usage.
For example, those who have the HTC Sensation 4G which will be getting ICS this week, once they upgrade and try to tether their phone for free, if T-Mobile is able to catch it, they'll simply be denied Internet access. Other carriers such as Verizon, AT&T, and Sprint, have had hotspot plans where for so much a month (usually around $20 extra), you can tether your device.
The update for the Amaze 4G doesn't say anything about tethering, but it's safe to assume that the same policy will be in place once the upgrade rolls out. It's still unclear whether non-standard tethering methods, popular with rooted users and those with custom ROMs, will be monitored as well.
It's another way of them to make money and while some may be upset about it, there's not much getting around it, unless of course you know how to and are able to root your smartphone or device. That is unless T-Mobile figures out a way to monitor those users as well. How I feel about it, you get so much data per month and should be allowed to access it how you want and not have to pay extra to get that same data.