(I'm working on Sourcing.io.)

While that is a possible outcome, we're working really hard to make sure that doesn't happen. We'd hate to be responsible for an increase in the volume of recruiter-spam sent to engineers every day. Although this isn't on the homepage, we are planning to add a set of tools so that engineers can manage their profiles and companies' technical recruiters can coordinate engineer-to-engineer outreach. We believe that it's a lot better when engineers should receive cold recruiting emails from the people they'd actually be working with.

So the answer is /yes/. Everyone you index has to know about you, and go to your website in order to opt them out of receiving emails they never asked to receive in the first place? That means it's spam.