The tapes are likely faked but the sentiment on them is genuine (whether it is the will of Osama or not, look at MLK III say that his dad was against the war in Iraq even though he died long before it was a concern). There are those in Al Qaeda that do want to see attacks on American interests. Look at the "lone nuts" that took action into their own hands after 9.11.01.
Plausible deniability is the name of this game and the fewer attacks that can be linked to Al Qaeda, let alone Iraq/Saddam, the more it eases the simple minds of the left.
Never have seen a trace of either Adolph Hitler or Jimmy Hoffa's bodies yet there is no doubt they are both long since dead.