Question about "branch.<name>.merge"

This is a typical section from a .git config file:

[remote "origin"]
    fetch = +refs/heads/*:refs/remotes/origin/*
    url = git://git.kernel.org/pub/scm/git/git.git
[branch "master"]
    remote = origin
    merge = refs/heads/master

I am curious why "branch.master.merge=refs/heads/master"  was
implemented to refer to the remote branch (refs/heads/master) rather
than the local branch (refs/remotes/origin/master).

Thanks.

Josh