If you put your public email on your github profile, or put your email on public issue comments, you may be contacted regarding news of the repository you showed interest to.
Like getting feedback, you know.
Personnaly I would be pretty pleased that a maintainer made that effort.
I can totally understand that others person consider that spam ... Well just don't make your email public in that case.
Someones e-mail being public does not give you the right to send them spam (=which I consider this, under the common definition of "unsolicited mass-e-mail"), and having starred a GitHub repo does not imply consent to that. If someone wants notifications, there are tools for this: they subscribe to your repo, individual issues within it or a newsletter.