It would be like posting part of a conversation that wouldn't make sense to anyone on your Facebook or Twitter.
I very rarely post anything locked, so I don't really care if your comments to any of my posts show up on Facebook or Twitter, but if I do not want it to happen for a particular post I will state so at the top and kindly ask you to respect it. Thanks.
[Edit: just tested out the whole comments from flock thing and they do repost, which seems utterly moronic to me, wonder if they'll fix that soon. I would hope so.]
[Edit: I do think that LJ should have an option on posts that allows you to refuse tweet/facebook repost of comments on your entry. A post is yours, it belongs to you alone, so it's yours to do what the hell you want with, but comments are often part of a conversation. By definition a conversation involves more than one person and so all should have to be in agreement for the conversation, even from only one side, to be rebroadcast in another medium. Just my two pennies worth :).]
[Edit: added my thoughts at the news post about it.]