Look at some other examples...
http://tmagazine.tumblr.com/
http://life.tumblr.com/
http://today.tumblr.com/
Compare the examples I listed above and see if your same concerns of ownership apply there as well...
life.tumblr.com - Clearly states that it provides an additional outlet for Life.com editors, above the fold if you will, along with an easy to identify contact point/editor
tmagazine.tumblr.com - Clearly states its affiliation with the Time Style Magazine as the first thing you see, and also provides obvious links at top and bottom
today.tumblr.com - Eeh, kinda in the air about this one, but they do clarly have the Today masthead, along with an obvious link to today.com directly below it
Tumblr is just the location/host - you don't assume Tumblr owns all the content in all the feeds do you simply because it's at tumblr.com instead of dreamfinder.com do you? Do you question a blog because it uses wordpress instead of something homegrown? Or how about company pages on Facebook? Or Twitter? See a trend here? The format/location is a compromise participating companies accept - they don't get to pull the full experience/benefit of that format/location into their corporate domain.
Except there is no trend here. Since you explicitly mentioned facebook and twitter, check out the facebook.com/DisneyPhotoPass and twitter.com/disneyphotopass. Both clearly state "Official page for Disney's Photopass Service". Ditto for facebook.com/vinylmation, "Offical vinylmation facebook page". And check out twitter.com/disneypixar, facebook.com/disneyland, facebook.com/waltdisneyworld, facebook.com/carsland, facebook.com/duffythedisneybear, etc. See a trend here? They all say Official Whatever for Whatever.
And as you said, when you read the content from the site, you assumed the content was approved by the site curator. That's exactly what's happening here and why addition 'clarification' is not only unwarranted, but over the top in expectations.
I don' assume Tumblr owns the content at all. But in almost every commercial Tumblr site, the owner is clearly shown. All 3 of the sites you listed, clearly show ownership smack dab in the top of the masthead. Others, such as boston.tumblr.com and latenightjimmy.tumblr.com, clearly state the source of the content, and in the case of Jimmy Fallon's site, even states that it is the official site of LTwJF. The Disney one? Has a Disney Parks logo on the bottom, and if you click the about, mentions Disney Parks. Thats all.
I happen to have 15-20 other Disney related tumblrs kicking around in my favorites, even one or two that appear to use a very similar theme and layout as this official one. They all have "disney", "disneyland", "waltdisneyworld" or similar somewhere in the title, and often contain their title in WaltFont (or whatever we call the Disney script now).
Much the same way that the UK courts smacked Apple's hand for not properly displaying their apology letter to Samsung instead burying it out of sight, get it somewhere obvious that the site is owned/curated/edited/whatevered by an official Disney mouthpiece somewhere instead of forcing the user to click on a link to see it.
That is where the biggest issue with companies trying to co-opt social media and even guerrilla marketing. At some point, you need to own the product. Claim ownership loud and proud, or the entire campaign is worthless. Stick a comment on the template next to the Disney Parks image/link that says "Curated by:" Voila. Ownership claimed, and easily identified. Not looking for an overhaul of the minimalist design, (which I am a proponent of) but a simply easily viewed ownership claim, falling in line with the other externally hosted Disney properties I referenced above.
Better yet? Take a page from the Official Disney Pixar Tumblr seen here ->
http://disneypixar.tumblr.com/ Scrolling down to the bottom, it does provide links to the official disney.go.com Privcay Policy, Terms of Service as well as a Copyright Disney 2012. They can do it, and normally do follow these practices, but for whatever reason have chosen to ignore them for this one site.