tracking branch for a rebase

I received an interesting question today.

If "git pull" knows what remote tracking brach to pull from,
why doesn't "git rebase" automatically know I want to rebase
on top of my remote tracking branch?

He had done a "git remote update", reviewed the changes, and now
wants to rebase his changes on top of the new upstream.

I told him about git pull --rebase, and branch.name.rebase,
but didn't have an easy answer to the "what is my remote tracking
branch" question in this case.

All the information is in git config branch.name.{remote,merge},
but I can't find anything that just answers that question.  Nor
did I have a good answer for why "git rebase" doesn't just default
to rebasing the current branch on top of the tracking branch.

Thoughts?

		-- Pete