My personal opinion is that the carriers don't enforce their tethering policies simply because it's hard to prove. It's one thing to say your data usage was extremely high for a month. It's another thing to say it's because of tethering. Excessive data usage will usually get you a warning or two before any action occurs.
If anyone is interested, here's the entire current Sprint Subscriber Agreement:
http://www.sprintpcs.com/common/popu...msPrivacy.html