My guess would be along the lines of the above comments. AutoCompletion is an inherit security risk, if someone accesses the site from a public computer, auto complete saves the username and/or password, then another user could login.
I personally don't believe it is much of an issue as long as you have educated users, from an admin side of things. However, many believe this to be a much large security risk.
I agree with the statements by Michael, this should be a configuration option, possibly in "user settings" where all other elements related to login process are stored.