I have an almost gut-level resistance to the idea of deliberately leaving a "back door" open in my computer. I believe this to be one of the areas where preserving a freedom is worth the increased risk.

Also, even if we assume the FBI and the government to be totally trustworthy (merely typing that sentence is giving me hives), any number of crooks becoming aware of the back door's location could take advantage of it.

Would you give your local police department a key to your house, permanently, "just in case?" Now, if our neighborhood was being burglarized and people attacked, and the cops wanted to stake out my house for a specific period of time and needed a key since I'm not always home, then I would be inclined to let them have one. I would, of course, want it back later. Even then I might eventually change the locks, something I have done several times in response to lost keys or keys that have gone missing for too long.

Similarly, I support temporary increases in FBI and police powers under the current circumstances, not uncommon in wartime, but we'll see if they evaporate on schedule a couple years from now. Unfortunately, I'm not sure a back door in a computer program can ever be "temporary." Maybe after the emergency is over the antivirus companies can legally add the protection back, but who would be sure?

Fortunately, from a libertarian standpoint, there will be no lack of wizards out there who will gleefully drop free protection against Magic Lantern all over the web. I'm not sure anyone will be able to stop them.