I don't think Verizon can FORCE an OEM to lock their phones down, but they DO reward OEMs for doing so... hence Motorola's preferred status.
I disagree. Verizon doesn't have to let any device on their network they don't see fit. They have 100% control over this. The question is does Big Red care? I think everyone is opening up to making phones available to the modding community with the expectations of limiting or voiding warranties.
Edit Addition: In regards to Motorola getting preference for locking their bootloaders, that also doesn't make sense to me. Motorola is the first vendor to release a device with an unlockable bootloader on the Verizon Network. If they are preferred, their preference could be linked in the quality and sales figures of their devices. Moto has had much better build quality over the other vendors. I own a HTC Incredible and it is an awesome phone but it feels cheap compared to the OG Droid. Moto has managed to have a solid handle on battery life compared to the the other manufacturers with HTC being the worst.