Normally, this would seem like exactly the type of viral marketing put out by AMC, but in this case it can be safely assumed AMC isn't behind it. Why? Because the network sent Twitter a DMCA request to shut the accounts down, according to Venture Beat. It took, according to Silicon Alley Insider, some "gentle prodding" by AMC's Web advertising agency Deep Focus to allow the Twitter sites to remain active. After all, if fans are crazed enough to microblog about the characters for the show, then why not let them? As of yet, none of the people involved have used the Twitter accounts in anyway that AMC could interpret as harmful to their property.
Which isn't to say that the takeaway is that every time a random person on the Internet starts appropriating a company's brand, the company should let them. As Alan Wolk over at The Toadstool points out, many brands would have much to lose by letting someone off the reservation take on any role as their spokesperson, as Exxon found out when someone created a fake Twitter profile as an Exxon spokesperson.
What exactly constitutes marketing is in flux, and this episode shows how some companies are struggling (and why it's great to have a good agency on your side).