It's easier to benefit from a false-flag attack by just doing it- denials don't carry as much weight in an emotional, post-attack context. When forewarning is given for a false-flag, you give the accused a chance to back away and say "it's not us" before an event, lodging enough doubt that the size of the crowd with torches and pitchforks isn't as big afterwards... I think this would be counter-productive for the guys staging the false-flag attack.