Article 15 (i.e. Nonjudicial) punishments are pretty accurately described. They are not judicial in that the burden of proof is not criminal, nor are the offenses, penalites nor proceedures criminal in nature and do not need to comport with any rigorous notion of due process, or he'd be before a court martial. What this clearly means is that Specialist (now Pfc.) Clark did not violate any laws, committed no crime, and likely didn't do any harm to anyone but himself. Instead, it means the Army has used the easiest means to impugn Clark's character with the added benefit of not having to stand their allegations up against any reasonable evidentiary standard.
There is a good reason for that; the information Clark posted wouldn't withstand that level of scrutiny. No military judge in court martial would have convicted Clark of any offense encompassing endagerment of operation security based on the very innocuous operational information Clark posted.
I have read many milblogs, and almost universally, the type of information posted them could just as easily be characterized as op sec relevant. Just saying you are going on duty (the entry is time-stamped, remember) is op sec relevant. Saying that you are guarding a particular facility. That mortar fire came in and damaged certain buildings, equipment, or personnel. That you heard that somebody from a certain unit was killed or injured.
All of these posts would technically compromise op sec. Hell, noting the time you have lunch at the mess would. But milblogs post this sort of information all time. If one were to painstakingly correlate all the information available over milblogs, it might constitute a valuable intelligence resource, even if no one blog or blogger were really endangering operational security. That's a serious problem that the military needs to address systematically, but it is not a justification for singling out Clark.
So why was Clark singled out for very harsh punishment (loss of rank) in the very informal process of non-judicial punishment, when hundreds, perhaps thousands of other blogging soldiers have done the same or worse? Clark openly opposes the war, and made the mistake of challenging one of the politicians who firmly backs this war. That's the reason that the Army has mugged Clark without benefit of judicial process and without any substantive evidence that he caused any harm.