Hollywood still doesn't get social media, so they are doing the only right thing (in their mind) banning it. Studio execs are so afraid that actors are leaking film information on Twitter and Facebook, that they've decided to insert clauses in their contracts forbidding them.
Disney-owned Dreamworks are one of the scared studios. According to The Hollywood Reporter Cameron Diaz and Mike Myers are affected by the new clauses. God forbid someone should make any free publicity for Shrek 5 or is it 6? (I've stopped counting).
This could also explain Miley Cyrus' Twitter exit earlier this month. The Disney talent contracts forbids confidentiality breaches via "interactive media such as Facebook, Twitter, or any other interactive social network or personal blog." Dreamworks are restraining eager writers with a similar clause in their contracts regarding sharing of press releases on "a social networking site, blog or other Internet-type site.”
Maybe Ryan Seacrest scared them when he announced Ben Silverman's resignation as co-chairman from NBC on Twitter before it was officially announced.
Or perhaps it was when Paula Abdul quit American Idol using Twitter?
It could also be because Greg Grunberg tweeted a goodbye to the Heroes crew. "Tough to say goodbye to crew not knowing if any or all of us will return next year." Fans instantly thought that Heroes wasn't coming back for another season until he tweeted " Heroes IS coming back but some crew take other jobs, so it's tough."
This is just another example of, why Hollywood is afraid of Twitter and other social networks. They still haven't seen the benefit of it. Hollywood wake up, you have no idea of the potential. I would worry more about making quality film.