It's not your browser's main focus to fix spelling mistakes. Even then, they've come a long way. I remember Netscape 4 not being able to handle "news.bbc.co.uk" due to lack of "www." in there. You had to manually add "http:// in front of it.
A friend replied in private mail: "Because it is not a valid host name (e.g http://www.ietf.org/rfc/rfc952.txt). My browser searches on an Internet search engine whatever I enter in the address field that is not a valid host name. I think that is a pretty good way of handling it. What does your browser do?"
