Reddit has found that certain websites or their operators spam links on Reddit and has banned these domains from being posted on the site. The newly created /r/BannedDomains subreddit details the sites whose links are banned from being posted. Users who try to post a message containing a link to a banned site will be denied with an informative message.
The list of banned domains contains some major sites and is as follows:
The reason behind blocking these sites? It turns out editors from the sites, some of which are highly influential, have been posting links to their site on Reddit in a rather spamming fashion. A link on Reddit can often garner the site many extra hits or views that would otherwise not have come in. These hits are monetized by advertising making the site more money.
Reddit explains the ban in a short post:
Some domains are not allowed on any part of reddit because they are spammy, malicious, or involved in cheating shenanigans. Attempting to submit a link to one of these domains will now fail with an informative error message.
It's reasonable to understand why someone would post lots of links to their site on Reddit. A top link can bring in over 100,000 extra hits and even a more minor link can still bring in 10,000 extra hits. It's not clear if the sites will ever make it back onto Reddit, but they need Reddit more than Reddit needs them.
Take this as a warning shot across the bow. If you want links to your site to be able to be posted to Reddit, make sure you aren't spamming them all over. Reddit wants to keep its site free from spam as much as possible.