What if the blogs are false?

Reading through some old editions of Media Guardian I found an interesting comment by Jeff Jarvis, a journalism professor and blogger at buzzmachine.com, with the headline. Buzz off! this blogger’s voice is not for sale.
Being a PR student I’ve learned that journalists are a bit sceptical towards PR practitioners, and in this comment Jarvis is set to reveal the cynical and manipulating practitioners on the web.

His idea is that the plan of blogs was that they were of ‘user-generated content’, but that they have spawned a new commercial practice that is word-of-mouth marketing: “The dark art of trying to manipulate the public to buzz about your brand”, as he puts it. He says that because it’s now easier for PR to publish its stories and avoiding the gatekeepers in the press, they should be aware that there credibility is more important than ever.

There are many examples of PR stunts in Web 2.0 that has gone wrong. These are arguably made by people that underestimate the intelligence of the public. One example Jarvis points out is one managed by PR agency Edelman:
A weblog supposedly written by two ordinary folks who drove their camper van across America from Wal-Mart to Wal-Mart, churning out heartwarming tales about working folks there, turned out to have been paid for and managed by Edelman, the retail giant’s PR agency.

It is stories like this one that suggest that PR agencies don’t now how to respond to the new challenge of the Web 2.0. Jarvis claims that the basic ethics of blogging should be telling the truth. Using Dell as a new example they claim to have good computers and excellent customer service, but searches on the web shows that the nickname “Dell Hell” is widely spread because of people’s bad experience.
If the idea of a blog is to create a buzz around your brand, the information that is published is better off if it is the same as if you google it. If the information doesn’t match, the buzz that you wanted will be bad buzz.

So, presuming that PR practitioners are aware that being caught in false marketing will injure the brand, why do some still take the chance?