No one (least of all at NSA) thinks AQ are brainless goons. That would, in fact, be why NSA wouldn't want to divulge detailed-enough "This is American data" guidance, as they assume that AQ and others would be able to play off that guidance to alter their own COMSEC procedures.
In fact forcing AQ to switch to using postcards would be beneficial in some regards from the NSA POV as it would slow their communications cycle. If you've never been in an environment where command-and-control are important, let's just say that effective communications are the backbone of operations. Many less-than-perfect debriefs I've been involved in had a major lesson learned "Poor Communications".
There's a maxim that most plans don't survive first contact with the enemy. From AQ's perspective that makes communications import so they can determine the problem, adapt and improvise a solution, and communicate that solution to all affected stakeholders.
Likewise even AQ engages in administrivia like expense reports, promoting leaders, org. alignments, etc. Those all need communications.
So when a break in AQ comms may rely on them screwing up a single time, it becomes even more important not to give away the "Do's and Don't's of NSA SIGINT". Even forcing them to always use GPG might be beneficial, with things like Geo-IP and metadata analysis.