it's because OP linked to the HTTPS version of reddit, which doesn't exist. akamai will serve an HTTPS version of any page that they cache, and if the customer has not configured HTTPS they will serve it up with their certificate.
reddit uses akamai for page content but not for any authentication, there are no security benefits to using https://reddit instead of http://reddit
there was an explanation from one of the admins once upon a time about why pay.reddit wasn't any more secure than standard reddit. i forget the details, but the consensus was don't bother.
reddit uses akamai for page content but not for any authentication, there are no security benefits to using https://reddit instead of http://reddit