The reason, I'm sure is to limit the search results, which makes a huge difference for the database server, and the presentation, as well as not showing hundreds of worthless results to the user.
If you search for "in" look how many results you'd get back, just from this paragraph in Lavalamp's last post:
However, back on topic. I think that the reason the min word length is 4 is because you can use "and" and "or" in the search box to be more specific. If the min word length was 3 then the server-side script might get confused between whether the user actually wanted to search for the word "and" or whether it is meant to be a joiner-togetherer word.
That's just one paragraph. Users could bog down the database to where the forum would be too slow for usability, if searches for things like that were allowed. Also, you wouldn't want to wait around for the presentation to figure out what to do with a few tens of thousands of results.
I don't work for webdeveloper.com, but I limit some types of searches in my own work for that reason.