My geuss is that they are concerned about blogs that indicate deteriorating morale. That information is of use to the enemy as feedback for the effectiveness of their operations. As in Vietnam, this enemy cannot defeat us on the field, they have to defeat us in our own minds. As a former intel officer, these blogs are what we used to call "indicators". Anything overly pessimistic or critical of the chain of command can be used in a variety of ways by the enemy (here and in Iraq).
And, since it come directly from the soldiers themselves, rather than through the filter of the press, it carries significantly more weight. What makes these blogs so interesting to us, is precisely what makes it potentially powerfull for use in counter-US propaganda.
There is a legitimate OPSEC argument here. Hate to say it, but it's so.
I see two likely options:
1. Trust 'em or bust 'em: Train thoroughly, monitor closely, punish those who violate opsec.
2. Flip the comm switch "off".
I propose option one. But I'm biased.