Can someone explain to me why is this more exciting than telling me the my browser history can seen by other people?
This is a kind of hypocrisy we really need to get over as a civilization. Almost every office worker browses Facebook, Twitter, eBay and equivalents for significant amount of work time. Let's just admit it and stop caring about it. Trying to fight it only destroys office morale. Private browsing is often an important part of maintaining a mental hygiene in the workplace.
On the browsing Twitter stuff, I'm kind of fortunate. Since this hit here and on Boing Boing, I've been following along (and have been pleasantly surprised at how nice the internet is). But, I'm in academia, and this very tangentially counts as "work."
But yes, I also agree, people are doing this, and they'll continue to do so at work. As for how that fits with your model of a productive workplace, YMMV.