The British MoD sees public activities, such as running blogs, podcasts or joining online communities as a potential security threat. This is no surprise, since irresponsible personnel could unwittingly divulge sensitive information which would be instantly visible around the world.
Apart from sensitive intelligence info, there's probably an even bigger threat to the army's image. Compromising images of men and women in uniform from several nations have recently surfaced on the net, causing an uproar, such as the video of German Bundeswehr troops shouting racial slurs during training.
Another recent incident comes to mind, the capture of British marines and Navy personnel by Iranian naval patrols. The sailors were quick to cash in on their ordeal, although the MoD itself chose to approve their book and TV deals.
The US Army already has strict guidelines to prevent security breaches, as well as a radio campaign aimed at raising online security consciousness on their armed forces radio network.