Can I fetch an arbitrary commit by sha1?

I always though during fetch I have to specify a refspec and that a
sha1 would not be accepted as a ref. Firing some like 'git fetch
origin <sha1>' should be forbidden. But in fact I see that such a
fetch command succeeds if you already have that object in your local
repo.

My question: is it allowed to fetch sha1's? Shouldn't fetch fail if you try it?

See here:

git clone -q https://github.com/chalstrick/dondalfi.git
cd dondalfi
git ls-remote

From https://github.com/chalstrick/dondalfi.git
ce08dcc41104383f3cca2b95bd41e9054a957f5b HEAD
af00f4c39bcc8dc29ed8f59a47066d5993c279e4 refs/foo/b1
...

git show af00f4c39bcc8dc29ed8f59a47066d5993c279e4

fatal: bad object af00f4c39bcc8dc29ed8f59a47066d5993c279e4

git fetch origin af00f4c39bcc8dc29ed8f59a47066d5993c279e4

error: no such remote ref af00f4c39bcc8dc29ed8f59a47066d5993c279e4

git fetch origin refs/foo/b1

remote: Counting objects: 3, done.
remote: Compressing objects: 100% (2/2), done.
remote: Total 3 (delta 0), reused 0 (delta 0)
Unpacking objects: 100% (3/3), done.
From https://github.com/chalstrick/dondalfi
 * branch            refs/foo/b1 -> FETCH_HEAD

git fetch origin af00f4c39bcc8dc29ed8f59a47066d5993c279e4

From https://github.com/chalstrick/dondalfi
 * branch            af00f4c39bcc8dc29ed8f59a47066d5993c279e4 -> FETCH_HEAD

Ciao
  Chris