Except advertising is typically declared as such. Look in a magazine or newspaper. Anything that isn't obviously an AD will typically have "Advertisement" or "Special Advertising Section" at the top/bottom of the page. Read a computer magazine, and you see a full page spread for Microsoft, you know immediately it's an AD. If you see a full page "article" about how great Windows 8 is and how it revolutionized someones life with no negative mentions, typically it will have the "Advertising" header, as it's not an unbiased story. This is why most editorial and advertising staffs are clearly separated to help ensure no cross over.
Social media, often has no such declaration. And this can lead to a biased viewpoint. Much in the same way that food critics often strive for anonymity, would you trust a restaurant review where the reviewer was known in advance so the restaurant could ensure that his meal was perfect and he received a non-standard experience that he didn't need to pay for? So why would a blog posting/WDW article/review whatever be any different? When being presented with a viewpoint, if that viewpoint is in a situation where they could feel biased or beholden to someone who compensated them for the viewpoint, isn't it important to know as such?
If something is as ad, declare it as such. Even in guerrilla marketing, it's easy to determine that it is an ad pretty quickly. How many people would walk down the street with cases of an energy drink out of the goodness of their heart? Or setup some waterslide temporarily downtown to advertise a new product. Stuff like this happens, but because people know it is out of the ordinary it is easy to assume it is advertising.
Social media marketing/whoring/plants/whatever aren't outside the norm. Someone on a WDW board pontificating about their favorite ride, completely normal and expected. So no way to know that a new poster is really a plant or someone astroturfing on behalf of WDW. Or on tumblr, there are tons and tons of tumblr's dedicated to WDW/DLR, where people post their own pictures, others pictures, etc. So there is no way to know that a given tumblr is really paid for and curated by the mouse, and not Joe Blogger in his basement. For instance, the tumblr this thread initially started about. Several of the pictures posted are in situations that a normal guest would never experience. Cinderella posing in the dream suite, or the shot of several characters just chilling on a planter. Since a guest may not know that those are not normal situations, they could go to WDW expecting being able to have time with Cinderella in a gorgeous setting, and not a cheap backdrop. Or the chance to get their kids picture with 6 characters at once. If these are clearly presented as advertising, most people would realize that this may not be a normal situation in the parks. But by posting it on a social media site, in a way that doesn't scream "Official" or "Advertisement", people may be mislead about the experience they will get to have in park. Admittedly this is a weak example, but it ties the whole social media debate back into the thread in hand. And I do believe that companies have been sued in the UK for less, things like indicated a electronic device turns on much faster than it really does.