Osama bin Laden knew full well the US government was spying on al Qaeda, long before Edward Snowden blew his whistle. In documents recently made public during a terrorism trial in the United States, bin Laden writes:
[J]ust because something can be encrypted doesn’t make it suitable for use. As you know, this science is not ours and is not our invention. That means we do not know much about it. Based on this, I see that sending any dangerous matter via encrypted email is a risky thing.
People engaged in criminal activities like terrorism know full well that government agencies are trying to tap and decrypt their communications. Disclosures like Edward Snowden's therefore serve to inform the general public, not the terrorists. The terrorists already know they are being watched.
Read more on the security paradox.