Most importantly, the NSA is not just about spying, it is also about protection.

A weakness anyone can exploit in software Americans use is not a good thing for the NSA. If they were to introduce weaknesses, they want to make sure only they can exploit them. For instance in the famous dual_ec_drbg case where the NSA is suspected to have introduced a backdoor, the exploit depends on a secret key. This is not the case here.

On the other hand if Snowden has shown us anything, it is that the NSA is more stupid than it looks.