A search really pounds the database so generally its a good idea to limit searches. A few people searching common keyboards could DoS the site.
These searches are simple pieces of code. Usually just a search for keywords going through every row of every table and returning the results. Considering its trivial to use Google to specifiy " site:boards.straightdope.com " there’s not much incentive to fix it. Even then you would probably want a dedicated computer to just do indexing for performance reasons. Why bother when google is doing it for you?
Note that there is a big difference between little-old-you doing an occasional search and Those Jerks who just search and search and search. Some are just clueless idiots, others are deliberate resource wasters, or and then there’s spammers looking for threads to place ads based on keywords. Hence the limits on time between searches that are so common. If they make it easy for you to search, then Those Jerks will take over and take it all down.
Wouldn’t it be great if forum search engines were mini googles? Sometimes I need to search for acronyms or three letter words but I can’t. Other times I want to use + and - to narrow down results but it’s impossible.
I could use The Google and specify the url via the site feature but it never searches through all the posts on the forum.