Git merge default?

Sorry that this is cross posted to git-users but I didn't realise how low
traffic that list was.

Why doesn't `git merge` default to the tracked branch in the same way
e.g. `git push` does?

Often I find myself performing a pull, and when I switch branch I'm
told that "Your branch is behind the tracked remote branch 'origin/
master' by 12 commits". However, despite git knowing where my branch
is based and how far away it is, I still have to specify `git merge
origin/master` to perform the merge.

It's been pointed out that another `git pull` will usually have the
same effect, but it seems that this introduces a redundant fetch, and
to be honest I couldn't work out whether the pull would fail if the
remote repo was unavailable for any reason.

Obviously this is a really minor point, and it doesn't exactly affect
my working day for more than a few seconds, but I was curious what I
was missing that such a complete package as git had this little
internal inconsistency.

TIA,
Gareth