For security and warranty-related reasons, much? Or to prevent people to randomly flash stuff they found on forums? I don't want to sound too aggressive but please try to consider the manufacturer's POW before bashing.
Anyhoo, I suggest you do a bit of research on "fastboot" and "adb", then when you think you have a basic grasp of the whole thing flash the update/radio etc.
Otherwise, you can search Google for "root Nexus S" and you'll find plenty of step-by step guides to unlock the bootloader..