Now we are getting somewhere. If vzw has a foolproof way to tell that you tether, and can whenever they wanted to, hit someone with thousands of dollars don't you think this would be a more effective way to put a stop to it?
Not the best approach to customer relations. RIAA and VZW are significantly different in terms of the direct relationship with customers and also competition (RIAA doesn't have to worry about customers going to another "carrier", it's an industry association rather than a single business).
AT&T sent out letters, so obviously they can and do have a way to identify tetherers. AT&T also threatened to hit them with charges or terminate the contract. I'm not sure why you're so hung-up on VZW not being able to tell this - every device has a unique MAC address so it should be pretty easy to tell when you tether. How else do you think they monitor actual tether data usage officially through their plan?