Doesn't a direct link cost more bandwidth? In which case does the one google ad on your site make the difference in cost/income? Further, does the ajaxian style hotlinking -- full credit and source link provided, constitute the same think most people think of for hotlinking (no credit, just images from someone else's site...).
He also does give an example of another site (Smashing Magazine) seeing the post on Ajaxian and believing that the work was the original work of Ajaxian (or a staff member of Ajaxian) and not the work of the real author, so it seems reasonable for him to be upset about this. Especially since his work seems to only be driving fame/publicity for Ajaxian, and not himself.