While you may consider user e-mail addresses to be public, some people take umbrage when their e-mail addresses are disclosed.
Personally I use a different e-mail address for almost every site, so searching by e-mail address wouldn't find my account, but I know many people use the same e-mail address for several things.
True, some people use a different address for each service (ie. yourservice@mydomain.com), and they tend to be the the kind of people one would want to hire as engineers! I wonder if Sourcing.io figured it out somehow, but at least compared to option 1 it seems better to look them up manually.
Personally I use a different e-mail address for almost every site, so searching by e-mail address wouldn't find my account, but I know many people use the same e-mail address for several things.