Personally, I'm horrified. But I seem to be in the minority.
1. This wasn't an agreement between Twitter and it's users and there is no opt-out feature.
2. How is this a LOC function? Shouldn't it be a Twitter function?
3. Is a tweet even a publication, or an agreement to do so?
4. What are the rights to privacy and/or criminal implications? Now that government can search the entire archive, if people have admitted crimes on twitter can they be punished? (Think college kids posting about weekend activities, e.g. drinking under 21 or other intoxicating pursuits.)
Now, if Twitter agreed to provide those tweets to LOC for everyone who opted in I might feel differently. I'd still question it being a LOC function though.