It's not justified and it can never be. Keeping information secret is only justified if releasing it would put a person in serious danger or compromise a police/military operation. And even then it must be balanced against the public interest in transparency, strictly time limited and narrow scoped (no blacking out whole documents).

I don't see how releasing an interpretation of a law could ever compromise law enforcement. Or actually I know how, it's because it would prove the way they do mass surveillance is illegal.

The problem is modern America doesn't see it that way. Any time the government does something evil they just say it's for "national security" and no one questions them. It's like a Winter Soldier password that makes courts go blank and say yes to anything.