Too bad, but I guess the checks and balances necessary to keep sensitive info (tactical, personal, etc) from being posted would be too daunting, especially 24/7 and the instantaneous nature of the internet.
Unfortunately, we can't assume that everyone will abide by the rules. Military members routinely give up their rights while protecting the rights of the many.
Welcome to the military. This is just another example of how the troops sacrifice their liberties and rights to protect the rest of us. I know it seems harsh, but the reality is that there are many occupations which, for various reasons, require that people not discuss their work or, in extreme cases, are isloated from contact with their loved-ones. It has also been the practice of the military for decades to control the flow of information to and from its people, for very obvious reasons.
While terrorists may be able to obtain reams of info. from the internet, we don't need to give them any help. I can tell you from experience, though it is extremely hard on those serving, it's much better than the thought that some careless remark might give the terrorists information which could lead to many deaths, both military and civilian. It's also for the serving-members' and their families' saftey: if terrorists are monitoring these communications, they could obtain info on where the soldiers and their families' live.
During WWII, letters home were all routinely opened and censored. Why would today's Army do any differently with e-mail?