There are good reasons to believe they may be lying about his death, though if he isn't dead you would expect him to release a video shortly...

One conspiracy theory is that America have identified that there will soon be attack, but they know they cannot stop it... so when it does happen they can just put it down to a retaliation for his death.

Another personal favourite (as I believe America were actually responsible for 9/11 but don't wanna cause a debate about that here) is that they can now potentially line up another attack and blame it on a terrorist retaliation (I don't actually think this will happen though)