MEMRI terrorist warning

The Middle East Media Research Institute (MEMRI), an ultra-zionist anti-Muslim propaganda outfit which specializes in publishing every single instance it can find of references to the 'Protocols of the Elders of Zion' by obscure Muslim clerics, has announced that the previously unknown - and we're seeing a spate of mysterious 'previously unknown' groups these days - 'Islamic Bayan Movement' has issued a communiqué, part of which states:

"Our Muslim brothers in America, we ask you to immediately leave the following cities: Washington, DC, New York, and Los Angeles. We are serious in our warning. The next few days will prove to you the truth of this warning. To the oppressive rulers of America we say: expect our terms following the first strike of Allah's believing soldiers."

What possible sense does it make for a terrorist organization to publish on a publicly available website information about a future attack which only a tiny minority of Muslims in the target cities will ever have knowledge of, but which American intelligence agencies looking for just this sort of thing will have immediate knowledge of? How much more obvious can a fake warning be? If you were going to warn members of a specific group about a planned terrorist attack, wouldn't it be much more sensible to do it privately, and in a manner that you know will reach your targeted audience, like, um, with an instant message service such as Odigo?