I agree. If I want merges, I run 'merge' explicitely. Also, something like this should make the useless merge commits I often see in git repositories (when they have users new to git pushing stuff) completely disappear.
This seems such an obvious change to me, though, that there probably is some good reason why it hasn't been done already...
One potential issue that I can think of is that after such a change it would be easier to rewrite history. However, most histories are unpublished anyway, and git-push already rejects actually publishing a rewritten history, which should be enough I think.
Also, it would be a kind of big behavioural change... If that is an issue, maybe the "git pull" and "git pull --rebase" commands should be completely separated to different commands?