I always figured the answer was that the information was for the US Taxpayer, and if other countries would like to use it. well, it is not for them, but if they can get access, fine, The effort required to try and keep them out is not worth it.

Sort of the same reason US government works end up in the public domain "This work was funded by the US citizens and as such it is owned by the US citizens(in the public domain), it is too much hassle to try and prevent non-US citizens access so if they can get a copy they are free to use it as well"

I liked the idea of having the intelligence agency maintain a public intelligence encyclopedia for it's citizens. Sort of how the geological survey provides public maps and earthquake alerts.