Looking at Issues 2.0: The Next Generation, this seems to be by design, and from the comments, this isn't the only "problem" users are facing:
It looks like issues can only be assigned to collaborators.
I'd still like to be able to assign an issue (or someone to claim one) to a developer who is not a repo collaborator. After all it is a very common workflow that collaboration happens with forks and pull requests.
One potential workaround (not tested myself) is for a user to fork the original repo, and reproduce the issue in the issue tracker of that new forked repo (that he owns):
- the new issue would keep an html link to the original issue of the original repo
- the user can assign and label issues.
Obviously that involves a bit of duplication, but for bugs a user wants complete ownership of, that can be worth doing.