The point is, people with a JB phone will have more problems, bringing their phone in more often costing Apple more money. Apple would prefer not to be required to do a factory reset to your iPhone.
Considering Woz jailbroke and Jobs is on the record making fun of him about it and seemed to turn a blind eye to it, my instinct is that the CARRIERS impose it. For instance, Thethering has been removed from the Verizon Android Market, but not the Sprint one, and Verizon says "Google did it, don't look at us!" and Google said "It violates Verizon's TOS!"-they have an agreement. Remember, when Apple came in, they had no phones, and were at the whims of the carrier-AT&T.
Honestly, I doubt Jobs cares about Jailbreaking, but he also likes Carriers selling and using his devices. If your phone was essentially a computer to do as you see fit, Verizon would deem it unsafe for their networks, for instance, and refuse to authorize ANY of them. AT&T may also try and block it from working.
Besides, the current Jailbreak Model saves Apple a LOT of R&D money. Why? They have a bunch of hackers constantly testing it for purposes that aren't malicious and won't lead to a scandal.
However, this whole scheme doesn't work unless Apple does make a stink about Jailbreaking. And they are concerned: If you have too much jailbreaking and everyone can tether without Apple seeing and downloading massive amounts of data to their phone in ways the phone is supposed to prevent, carriers might blackball their phones crushing their business model.
And frankly, 1% of users of a phone tethering probably isn't enough to start a problem. Look at Verizon, for instance: They know people tether, and they make it difficult, but this is also an illusion, as they don't enforce it. Why? With meters they'll get their money in the end. AT&T on the other hand is enforcing, and that proves that if Verizon really wanted to, they could, too. The idea with Apple and Verizon seems to be, as long as they keep the numbers low, its perfectly alright, when it comes to such matters.