The BBC is one of the latest groups seeking to ban or limit Twitter use by talent & staff. Their argument is similar to what a number of sports groups have used - they don't want early leaks to reveal sensitive information that might spoil their marketing plans or reveal information that may reduce the value of their products. The proposed policy would focus on limiting premature release of information such as storyline spoilers, casting news, or breaking news - information they want to control releasing, in order to maximize its impact or value to the BBC.
A classic hometown example of this rationale was the Southeastern Conference's plan, in 2009, to ban social media use by fans in stadiums - they feared that revealing scores or highlights might reduce the value of their football television contract. After two days of ridicule by press and fans, they reversed themselves, only "discouraging" fans from posting video clips during the game.
Still, concerns about controlling information flow are driving many groups and companies to develop policies to limit in-house use of Twitter and other social media. A recent survey of businesses in Britain showed that almost half have limited social media use while on the job; the most commonly cited concern (45%) being fears about business reputation. Similar concerns have driven may sports teams to develop social media use policies for players and other personnel; while not outright bans, the goal is to restrict certain types of information from being leaked. It's even gotten to the point where politicians try to ban social media from events (a recent example being Britain's Royal Wedding). A more extreme example is the efforts of several nations (Egypt, China, Syria) to ban social media, particularly during times of social unrest, and the bizarre case of France telling broadcasters not to use the words "Twitter" or "Facebook" on the air.
One of the benefits, or curses, of the modern digital media world, is that it makes controlling information more difficult - not impossible, but difficult and costly. Losing control over the message is a limit on power, and it's understandable that those in power seek to maintain and enhance it. But what limits power to one group can also be rephrased as a shift in power to others, to their benefit. Today, and going forward, the ability to bypass gatekeepers and those who seek to control information flows is ubiquitous - the genie's out, and those who used to be able to compel message control will need to turn to persuasion and gaining the cooperation of participants. Which, I think, is a good thing.
Sources: "BBC acts to stop Twitter leaks by stars and writers," The Guardian
"Companies ban Twitter from workplace," The Telegraph
An old academic paper of mine that addresses control in information systems - "The Macrosocial Impact of Communication Systems: Access, Bias, Control"