I am inclined not to believe that it has anything to do with service and support. I do however believe that with rooting and jailbreaking it opens the educated user up to additional functionality that could be circumvented thus costing them money. So at end, it's all about money.
I have a Nexus One, and of course its rooted, but with a rooted version of the N1 you could tether before Froyo was even available. At the moment Tmobile hasn't added a charge to it, but I am certain that depending of the usage and data rates that could change too.
I saw an article about the new firmware update for the iPhone and how they are closing the loophole which allows you to jailbreak the iPhone, and it bothers me to no end. If I purchase the phone, I should be able to do with it what I want with the understanding that if something happens, it is on me.
Apple started and was founded on the notion that they could make the PC better, but it was that sense of intrigue that motivated them to crack open a device predating 1976, that spirit still lives in people who want to test the limits of there skill, knowledge and ability. That shouldn't be withheld from them.
Release the source and kernel code, and let the legitimate not "hackers" because of the negative connotations associated with the term "hacker" but the legitimate computer science enthusiast go for broke. (No pun intended)
What Steve? Afraid some of these guys might show your $250,000.00 plus a year coders up?