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.
The Pandora's box has been opened, and even if Blizzard contacts YouTube to remove various kinds of media, and files injunctions against every single wiki and blog out there that does post the information, that box can't be closed again. There are a number of reasons Blizzard wouldn't want this information getting out -- first and foremost, it's just not done yet, so even talents and abilities that are listed as working in the alpha may be changed, reverted, or even completely removed in the final client. Second, releasing this much Wrath information at once hurts Blizzard's ability to control their player base's expectations and keep them interested in the expansion (we saw this at BlizzCon last year -- Frank Pearce was pretty unhappy that his keynote had been leaked before he got a chance to actually give it, because things he expected to be surprises actually weren't). So there are quite a few reasons why Blizzard wouldn't want this information out there.
From this side of the fence, Blizzard's feeling that the leaks are "offensive and inappropriate" isn't quite justified -- if anything, this news has gotten players even more interested in the expansion, and the information that has leaked seems just outlandish enough that no one is giving it too much heed. Even if the talent notes said that Druids were getting an octopus on a motorcycle form, that information would inspire much more player fervor if it was announced by Drysc on the forums rather than leaked out on a wiki.
In the end, the biggest result of this may just be for Blizzard to hold their cards closer to their chest for longer -- most developers realize by now that what used to be an "open beta" for MMOs is really a playable demo, and that you can't really let anyone download anything unless you want it to show up elsewhere on the internet. And "Friends and Family alphas," at Blizzard, will probably become a thing of the past -- while they'll still need playtesting done, it'll be by inviting people to their headquarters, not by sending out clients for download or through the mail.
As for sites covering Blizzard (this site, WoW Radio, MMO-Champion, World of Raids, and others), these leaks aren't coming from them at all. When asked by Blizzard to remove information, we did so, and many other media outlets around the WoW community have made the same decision we have: to not host Wrath information. There is a concern by some of these outlets that posting the information would cause problems with Blizzard regarding official access to coverage of events like BlizzCon and the WorldWide Invitational, but just like Neth's "offensive" comment, any action Blizzard takes against these sites is misdirected -- the leaks are the result of the people who signed the NDA and received (and then shared) the alpha client, directly against any agreement they made.
This will all be moot in a few months, of course -- eventually, we'll all hear plenty about Wrath and all the talents and abilities that will be open to us in the expansion. Whether you visit the leak sites or not, you'll know what's in the expansion even before you make it to 71. But until then, the only thing really "offensive and inappropriate" about this whole situation is Blizzard's attempts to re-hide what they sent out into the public in the first place. If they're looking to blame someone for sharing pre-release information or messing up their marketing plan, they should look to the Friends and Family they shared the alpha with.