Anybody can flag at any time. It only takes 4 sad people for a thread or comment to achieve flagged status. As an example anything with negative commentary about Julian Assange is instantly flagged because there is a cult that hangs on his every word.
It's a mechanism that makes sense to prevent flamewars and a lot of silly political discussion; however it's also an invaluable tool for anyone who doesn't want some topic discussed at all.
