I quote (from How do comments work?):
Who can post comments?
All users may leave comments on their own posts and any answers given
to their own questions. Users with at least 50 reputation may comment
on any post. (There is no reputation requirement to post comments on
See also: http://meta.stackoverflow.com/privileges/comment
By setting a (quite low) threshold, we keep out the spammers and the general 'me too!' comments that would flood the site otherwise.
Yes, people will still try and circumvent that, but the initial filter does keep most crap out. If you see such 'answers', just flag them with the
Not an answer flag and a moderator will come along shortly to clean these up.