The link only appeared to work for you because it was in your browser's cache. You visited the link (via Google Image search or whatever) and the image loaded successfully. When it was loaded it was placed in your browser's cache. That means that if you went back to the same site to view the picture it would load from your local computer rather than loading from the website's server. So, when you posted a link to the image on the forum here it appeared to work for you. However, when someone else tried to view the image in this thread they would get the "no hotlinking" image because they didn't have the image in their browser's cache (so the browser tried to load the image from the website's server and it detected hotlinking). If you had either cleared your cache or done a full page refresh (Ctrl + F5) you would have seen the same "no hotlinking" image.
An easy way is - if you link to an image from a site that you suspect may not allow hotlinking, before you send the post hit 'preview' then press Ctrl-F5 and see if the pictures refresh properly.