Suppose I opted to protect my tweets and you could only see them if I let you follow me. Even a search engine couldn't see protected tweets.
My protected tweets would not be in this LOC database.
But... if you retweeted my protected tweet, it would be viewable by all and thus databased. There's a huge privacy concern with that.
Unrelated, if I delete a tweet of mine from my own stream after I send it, would it still be archived?