There's probably a couple of reasons that Jobs + Apple are motivated to block jailbreaking.
1- The AT&T contract probably requires it, as they are the ones with the most to lose. A jailbroken phone can run homebrew apps and features that they don't get a piece of. And features they might not want you to have, such as tethering or unmetered phone calls, or downloading media from unapproved sources, whatever, need to be blocked.
2 - Apple doesn't want potentially buggy iPhones out there, as it reflects badly on them even if wasn't their fault. The whole reason that Apple products are more stable than, say Microsoft's, is Apple has complete control over the hardware, and significant control over the software. Hack-apps that were to catch on but either are buggy themselves, or aggravate bugs in iOS, can make Apple look bad-- and of course, to Steve Jobs, image is everything, just like with any other narcissist.
The only question is, can they do it, can they really lock out jailbreaking? The answer is probably yes, eventually, though it could easily require a hardware upgrade or two before they get there.