I don't think any number of letters or petitions is going to change AT&T execs' minds. I also don't think that locking the bootloaders has to do with warranty replacements, after all, whomever wants to flash different software are still going to try to. If they really were looking to reduce the number of replacements due to bad flashes, the software would be as open as possible so there wouldn't be a need for workarounds and custom recoveries, which is where people usually mess up. You messed up your flash? Just download the original software from AT&T, put it in your microSD, turn the phone in this way, and press reflash. No need to send for repairs.
I think it has to do more with the culture of control that AT&T has, either that or AT&T and Verizon keep daring each other to fu/&%k more with their costumers. It goes with forcing a data plan even if you buy your own smartphone from Ebay, rather than just letting you pay by the Kb. Same as forcing stupid and redundant apps (contacts, navigator, locker) and making then not removable in the stock firmware. And putting their logo everywhere on the phone, including a big animation on the boot screen that you cannot remove to let you know that you're using their stuff.
It may also have to do with advertisement and collection of personal info. A custom ROM would probably not send the personal info they rightfully deserve, you know, after also getting our money (or so I would hope).