Destructive pre-commit behaviour and "--all"

Why doesn't "git commit --all" keep the files added to the staging area, so "git commit --all" would have the same behaviour as "git add --all && git commit"?

As described here a pre-commit hook which changes files, when run with "git commit --all", will make it impossible to see what the hook changed.

Making "git commit --all" and "git add --all && git commit" have the same behaviour (or at least having that configurable) would allow looking (and reverting if necessary) the changes generated by the hooks.

- Jan