The original owner of pitchfork.tumblr.com has his own response with a screenshot of an email from Tumblr stating that they didn't ask him before reassigning it:
http://tumbledore.tumblr.com/post/393276231/this-is-in-respo...
http://tumbledore.tumblr.com/post/393276231/this-is-in-respo...
Just because they can, does it mean they should have? It doesn't seem like a fair assumption that the account is abandoned if there are a few posts form the past year, especially when Tumblr didn't contact the user (assuming they did not).
It sucks, but you get what you pay for.
I can't say that the original owner has made the best case for himself. Throwing around terms like "libel" and "exercise my right" is off-putting and makes it hard to empathize with him.