As copyright holders, they certainly have ownership of any notes, updates, or models released, so it could be considered a violation of copyright law to rehost the information. But to really get to the center of the problems behind this leak, you have to realize where it likely came from: the Friends and Family Alpha release. A little while ago, Blizzard distributed the client for Wrath to a number of their "friends and family" -- probably a group that consisted of employees and their WoW-playing relatives, as well as likely some folks at Vivendi and Activision, other Blizzard partners like Upper Deck and Figureprints, and probably a number of high-end raiding guilds who've done a lot of pre-beta testing for Blizzard before. Somewhere in there was someone not quite as trustworthy as Blizzard thought (odds are that Blizzard at the very least collected a signed non-disclosure agreement from everyone that they sent the client to), and that person uploaded or otherwise shared the client, against the agreement.
Cut to a few days later, after that person shares it with another person, who shares it with one more person, who then uploads it to a popular bittorrent filesharing site, and suddenly everyone's got access to everything in the Wrath client, and even if they can't play it, they can still datamine and grab information.