It's this way by design. I can only guess/speculate at the reasons.
Best thing you can do is to archive posts for subsequent searching. http://www.snapfiles.com/reviews/WinHTTrack/httrack.html
I would guess that admins would still be able to search them though.
Among the reasons could be:
1) Additional load on the server (searches are resource intensive)
and costs not warranted or relevant to the content (computer hardware)
of the site.
2) ?Encourages further topic creations instead of a recycling of old posts.
3) Not all past political opinions should be searchable.
On a side note, you can't even use Google to pull results on that section of the forums which is unusual.