>It looks like they gave him full credit for this. Unless he was selling access to this blog post, I'm not sure why this is an issue.

Perhaps you missed the quite important point that Ajaxian was using this post to drive traffic to their site effectively and selling advertisements based on its, and not their own, merits? They're not just linking to something and showing it to their users, they're getting his paid-for account to serve the content so that they can make advertising revenue off of his work. I think the ethical issue with such an action should be clear.