I recall running into the odd one or two like that - I have vague memories that it was because searching was _slow_ and it didn't take more than a few people using it to bring the site to its knees. Restricting it to logged in users helped, and I think I saw some sites restrict searching to only post bodies for a period too, for similar reasons.

I haven't run into it recently, but I have at least vague memories of it happening in the past.