It's the same reason I don't want "the good guys" to have decryption keys to my messaging service, because even if I did trust the FBI, the fact that there is a backdoor at all means it could be exploited by someone I don't trust
Again, if you don't want to use a device that has a locked bootloader, don't buy it. I fail to see how this business model should be legally foreclosed upon. You'll always have the option to buy a device that can be unlocked, someone will always sell such a device. But if you can't lock them, then I can't buy one even if I want to