Root is generally bad for the manufacturers / carriers....by enabling root access, any rogue program running as root can esentially hose the whole phone. It's basic UNIX security - root is all powerful.
Not to mention, allowing root to something like a tethering program takes the ability to control that service out of the carriers hands, who might want to charge a premium for a service. Sure, they'll allow you unlimited data from your handset - but granting you access to turn your device into a WiFi router is another service all together. Both VZ and Sprint offer specific, non-phone devices targeted for that purpose - with their own, seperate data plans. By allowing users to use any device as a WiFi router, they significantly hamper their ability to sell, and make money of the devices they've built for that purpose alone.
That said, if the carrier puts out a program that enables a WiFi router capability on your phone - they will want to be able to track and charge for the use of that service.
It's all about making $
Thus - "rooting" voids warranties...
(Don't get me wrong, i'm still gonna root mine as soon as someone figures out how)