They want search engines to index their profiles and provide organic search results links to their site, but then those same sites will require you to sign in when clicking a link to another public profile. You can search for that 2nd profile in Google and then view it without signing in, but not by clicking internal links. I've experienced this with Quora, LinkedIn, Instagram, FB and others. They want to have their cake and eat it too.
As a user of LinkedIn, I can pick which portions of my profile information I would like to be publicly available. This is not by default, so most people do not have it public. You can try seeing my profile without logging in. :-)