git push upstream - why do I have to git push --set-upstream origin ?

I created a local branch for testing Solaris and Sun Studio. I then pushed the branch upstream. After committing a change and attempting to push the changes:

$ git commit blake2.cpp -m "Add workaround for missing _mm_set_epi64x"
[solaris 7ad22ff] Add workaround for missing _mm_set_epi64x
 1 file changed, 5 insertions(+)
$ git push
fatal: The current branch Solaris has no upstream branch.

To push the current branch and set the remote as upstream, use

    git push --set-upstream origin solaris

Why do I have to do something special for this?

Is there any reasonable use case where someone would create , push the to remote, and then claim a commit on is not supposed to be for ?