You didn't read your terms of use very carefully, then:
http://www.t-mobile.com/Templates/Popup.aspx?PAsset=Pln_Lst_DataPlan
Your Data Plan is intended for Web browsing, messaging, and similar activities on your device and not on any other equipment. Unless explicitly permitted by your Data Plan, other uses, including for example, tethering your device to a personal computer or other hardware, are not permitted.
Otherwise, I'm done with this discussion... I still maintain that they're all pretty competitive when it comes to rates (yes, T-Mobile's voice plan is cheaper; for fewer minutes and fewer features), but what do I care? I have no intention of changing phones or carriers anytime soon (crappy old Samsung phone on AT&T for me!)... I've wasted too much time on this already.