This would make it trivial to spam every user of GitHub though, by first spidering the site for public usernames.
Re:Keep Your GitHub Email Address Private
by JohnTHaller on Friday August 09, 2013 @04:55PM
(#6188427)
Which would result in quite a lot of spam sent via GitHub's forwarder, which would then be marked as a spam server in Gmail and other providers, which would result in GitHub's server being blocked.
Saw this problem in the bootstrap project where we still can't get in touch with ~10% of users to get them to approve the addition of an MIT license:
https://github.com/twbs/bootstrap/issues/2054