Proponents of invasive government have a laundry list of things they want us to accept. Even if a factor on that list had nothing to do with this incident, it makes perfect sense from their point of view to use the opportunity to get action on their list.
Therefore the question changes from, "What national security interest is served by having the public believe that Osama used encryption techniques he didn't use?" to, "Whose interests are served by getting action against encryption techniques?"
Now to us it may look stupid, but consider this. Suppose you work in the Secret Service. Suppose you have spent your life doing background checks, tracking crazies who might attack the President (he gets an average of something like one attempt per day), locking down areas to block more serious possible assassinations, etc. Now suppose that you run across an article named [link|http://www.infowar.com/class_1/BELL1.html-ssi|Assassination Politics]. (That is the short version. There is also a [link|http://zolatimes.com/v2.26/jimbell.htm|longer version] out there.)
How much harder would it make your job if that protocol materialized? After all if the people funding Osama etc could just anonymously add to the price on the President's head, wouldn't they?
If you can prevent encryption from becoming widely accepted, you can significantly reduce the likelyhood that this nightmare will become a viable political force to reckon with. Does spreading distaste and distrust of encryption then become a reasonable covert national policy?
Now I obviously don't know if the above is what is motivating the government. But I have seen the kind of precautions they take when the President is coming through. A lot of people are involved. There are a lot of groups in the government whose job is to come up with possible nightmare scenarios, and decide on appropriate responses. Whether it is for that or another reason (see Echelon), I have little problem believing that discouraging the wisespread adoption and acceptance of general cryptographic techniques is something that many dislike.
Cheers,
Ben