And in a more generic way, therefore whatever the site, there's three possible reasons behind suck asterisked links:
You must be registered to see the links and aren't ;
The links are dead for a reason or another ;
The site is suspicious and shouldn't be used.
This approach is used because it permit to treat all the links in one go. It's the server itself that invalidate the links, therefore there's no need to search, then edit, the thousands post with one of them.