The bootloader is the first bit of code that runs each time you turn on your Android device. It is what launches all the other parts of the OS, and often times the easiest way to load a different or hacked version of Android is to tell the bootloader to load a different set of files. A locked bootloader won't make hacking the KFHD impossible but it will make it harder to do. Rather than simply start cobbling together an Android firmware to install, the first step the hackers will need to undertake will be to figure out how to get around Amazon's restrictions. That's an issue they didn't face with the original Kindle Fire.
Note that this problem with the locked bootloader is not uncommon in Android and in fact the Nook Tablet has one. That merely slowed down the hacking; it didn't stop it, and the same goes for a number of other Android tablets and smartphones. So I wouldn't get too discouraged at today's news. But when you add this to the fact that Amazon won't let you use replacement home screens, hides competitor's reading apps, and won't let you do anything about the lock screen ads, it's a pretty good sign of where Amazon plans to take this tablet. Clearly they feel that it belongs to them, not you.