While not explicitly saying "we take the blame," Verizon has issued a statement listing the reasons behind the locked bootloader. Said statement takes at least 99.9 percent of the blame.
"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."
It's interesting, because this sounds a lot like the same sort of reasoning behind AT&T and its original policy for not allowing sideloading on its Android devices. That policy went the way of the dodo once Amazon.com opened its Amazon Appstore, as it made AT&T Android devices the only ones that couldn't use apps from Amazon.com's then nascent alternative Android marketplace.
Still, the Galaxy S III is the latest of Samsung's Galaxy S series of superphones, which have been characterized as the Korean giant's flagship device for years. It's too bad that Verizon's version is just a "tad" less "super" than the same device on other carriers.