When you are using a social network, you are talking about what you are doing. When you are in the military, that typically is not something you want on the internet.
Seems to make sense.
That's really not an argument. It's obvious that the marines are trusted not to do that, because there are a million other methods of communicating on the net (it was made for that, right?) and if a marine wanted to, they could leak information quite easily. The only effective way to bar them would be to just not give internet access.
The main method seems to be email, and if they're similarly restricted to DoD mail servers, they can apply censorship on the server side.
Before the time of the internet, letters home were routinely censored and redacted before being sent.
I took away from the article something more to the effect that the Marine Corps is protecting their network (and the extremely sensitive information on it) from malicious software which may get on one of the computers after some unsuspecting soldier clicks on an innocent enough looking link. If I was a hacker intent on getting into the military network this is exactly how I would do it.
It says they placed a network wide ban on them, I wonder how tight their security is. Can it be bypassed with a proxy or VPN? It seems the real issue is "...links to malicious sites in the hope a marine on R&R ... might click on them", they would be better served by locking down the systems that are accessing the Web, having an IDS and educating the soldiers on cyber risks.