This thinking is misguided. Prohibitive or draconian approaches to controlling social behaviour are doomed to failure (think alcohol prohibition in the '30s or the ongoing "war on drugs" as examples). Moreover, it creates an unnecessary and unfortunate adversarial relationship between groups of people who ought to be working towards a common goal.
This same issue exists in business. People use social media daily - it is a part of their lives (for better or worse). In fact, many employees use social media to do their jobs. Consequently, prohibiting use of social media outright creates an artificial problem between management and employees that need not exist. Moreover, it suggests a lack of trust by management of employees.
While it is true that leaking confidential internal information or "dissing" the boss is undesirable, there are other ways to handle these kinds of situations.
Knowing that people will talk, gossip, bitch, whine, etc. (as they always have done) and recognising that people have new and highly-social channels to exploit is a good first step. In light of this, every business needs to prepare internal guidelines around social media usage.
These guidelines need to be produced before any issues arise and ideally should involve employees as part of their development.
Why involve employees?
The end goal is to have sensible and even beneficial usage of social media as a part of everyday business practice by employees. For this to happen, there needs to be buy-in from employees so that they embrace and support the guidelines through advocacy and management of their own behaviour.
A business also benefits from having clear, positive guidelines as they provide clarity around social media usage so that if issues do arise there is a place to start talking.
In terms of the Samoan Rugby Union, the point is somewhat moot as their World Cup campaign is over. I just hope that next time they take a little time talking with the team about how social media might be best used.