A letter sent from Verizon to the FCC earlier this month provides further evidence that the carrier supports OEMs locking bootloaders, while also outlining the reasoning for this controversial stance. The letter was written in response to a customer complaint that having his phone's bootloader locked was in violation of the Block C license for Verizon's LTE network, which includes "open access" regulations preventing carriers from limiting or restricting a customer's use of software while on the network.
According to Verizon executive analyst Fred Powell, since an open bootloader could lead to modified software, keeping them locked allows the carrier to maintain a higher level of customer service and support — if a customer has a problem, it's difficult to point them to a solution when you don't know what software they're running. Verizon's explanation may sound reasonable, but it doesn't actually tackle the original complaint as to whether or not locking bootloaders violates open access regulations; instead, the carrier seems intent on avoiding the question all together.
While a number of OEMs, including HTC and, yes, Motorola, have announced plans to stop locking bootloaders, it still happens at the request of carriers, as was the case with the Droid RAZR. And if Verizon's response here is any indication, it doesn't look like it will stop any time soon.
Update: Revised post to highlight how Verizon didn't address the Block C question.