This is my theory. Please dissect.
Adding encryption to a device also adds cost and risk to the development of that device. The only way a company would do that is if they were compelled to; if there is money to be gained by adding the feature, or if there is some other external impetus.
Considering the former, I can't fathom a way that Motorola would directly make more money by locking down a phone. They aren't going to differentiate the product or sell more units by locking it down; and I can't think of any proprietary SW on the DX that they don't want people getting there hands on - unless they consider motoblur a crown-jewel? (If you know of a way locking would help Moto sell more phones, please enlighten me )
So, if we assume Motorola didn't have significant internal incentive to add cost/risk to the DX project, the next stop on the follow-the-money-trail is the carrier: Verizon.
Does Verizon have anything to gain by disallowing root-access? Is there anything that root-access can provide that would reduce Verizon's profits?
Verizon charges $15-$30 per MONTH for the right to tether. If root-access allows people to do the same for free without detection? That sounds like a pretty solid financial incentive to me.
Verizon: We want to charge for tethering.
Motorola: That will cost you a little extra.
Verizon: How much?
Motorola: Not much, relatively speaking.
Verizon: <evil laugh>