User blog comment:ScoobycoolDoo/Who's the mole?/@comment-75.42.211.131-20111024025656/@comment-1038387-20111122083554

It's easy to see why people would fall for this:
 * Artemis is a villain in the comics
 * She was introduced later in the series
 * It's immediately made clear by Red Arrow that she's not who she says she is
 * Ra's also mentions they have an operative in her debut episode
 * When she lost her memory, she thought she was working for her dad
 * It was Sportsmaster (who we can safely assume is her dad by now) who revealed the existence of an inside source

People who haven't read the tie-in comic lack the info necessary to completely rule her out, though even without them it should be pretty obvious she's a red herring.