The cold hard facts: Verizon's Galaxy S III comes with a locked bootloader which, in the carrier's opinion, would "negatively impact how the phone connects with the network" if it wasn't. We reviewed the AT&T and T-Mobile iterations, both of which come unlocked, and experienced no network problems whatsoever. So…
Verizon basically admitted it is responsible for throwing down the bootloader gauntlet in a statement to The Verge. But! The carrier has done a 180 by putting the blame squarely on Samsung's shoulders via Twitter: "I'm sorry for any frustration. However, the Bootloader is locked per the Manufacturer. We cannot comment on other provider^EW."
Luckily, the device is still rootable thanks to the Android developer community. It raises a pretty big red flag, however, when the one carrier out of AT&T, T-Mobile and Sprint, inexplicably releases a locked device.
Samsung has nothing to gain from locking the Galaxy S III, so it looks mighty suspicious on Verizon's end to be calling the Korean company out, particularly when the device is unshackled elsewhere in the U.S. and around the world.
UPDATE: Verizon has now taken the blame for the situation:
Verizon Wireless has established a standard of excellence in customer experience with our branded devices and customer service. There is an expectation that if a customer has a question, they can call Verizon Wireless for answers that help them maximize their enjoyment and use of their wireless phone. Depending on the device, an open boot loader could prevent Verizon Wireless from providing the same level of customer experience and support because it would allow users to change the phone or otherwise modify the software and, potentially, negatively impact how the phone connects with the network. The addition of unapproved software could also negatively impact the wireless experience for other customers. It is always a delicate balance for any company to manage the technology choices we make for our branded devices and the requests of a few who may want a different device experience. We always review our technology choices to ensure that we provide the best solution for as many customers as possible.