Because the DS / media / gov is attacking it.

If it was fake, and the DS knew it was fake, they would want as many people to believe it as possible, because when nothing ended up happening on/before the 20th, it would hugely demoralizing to anyone who believed it.

But they are attacking it, because they know the more people that know about it, the more open/welcoming they will be of anything that does happen, and because it shields people from their propaganda.

Just my perspective.