The NSA must rely on a lot of open source software...

> Why couldn’t open source licenses be amended to forbid use in surveillance devices?

Because otherwise, it would be non-free.

If you look at the Free Software definition (from http://www.gnu.org/philosophy/free-sw.html ), the first freedom is : "The freedom to run the program, for any purpose (freedom 0).". "For any purpose" is really clear, you can't forbid someone to use it in a surveillance device.

The DFSG (Debian Free Software Guidelines, http://www.debian.org/social_contract#guidelines ) are even clearer : "No Discrimination Against Fields of Endeavor - The license must not restrict anyone from making use of the program in a specific field of endeavor. For example, it may not restrict the program from being used in a business, or from being used for genetic research.".