The bootloader is most likely proprietary, even if its Open source, it's still there for a reason - its function is to load the Linux kernel and Android. It is generally locked to stop people who don't know what they're doing from doing things like 'I wonder what happens if I delete /system folder - and then doing just that!'. You can generally unlock the bootloader easily enough by requesting an unlock code from the phone manufacturer (I've done this several times with HTC, Sony and Huawei, and always received an unlock code that worked).
You do need a reason to unlock the bootloader - generally to root/install a custom recovery and non OEM ROM like CyanogenMod.