Because its unprecedented thats why. BTW, I'm not partisan hack if you care to read some of my other posts.
Your logic fails me, but perhaps I was jumping to my own conclusions. And while its unprecedented or at the very least rare, that doesn't instantly mean its because the SecDef doesn't trust the Soldiers that's where your logic fails. You're looking at the end state which could be the result of several different reasons or lines of thought, BUT assuming the worst reason without any basis.
For example:
Maybe a security staffer was overzealous, that could result in Marines being disarmed.
Maybe the official reason is the exact reason, that could result in Marines being disarmed.
Maybe the SecDef personally didn't trust the Marines, that could result in Marines being disarmed.
Maybe a staffer was thinking of PR for back in the states and thinking a picture with armed Marines wouldnt look nice, that could result in Marines being disarmed.
See there are several ways to reach the current endstate which was Marines being disarmed, why are we jumping to the one without any basis and ignoring the one that has basis? And why does the conclusion we are jumping to, again the one with no basis, also happen to be one the one most politically damaging for the administration? It has the most politically rhetorical value, to me that says because many of the people in this topic are looking for a way to attack the administration instead of looking for facts. I'm a man of facts, politics means nothing to me.
Now I can understand why it would make people feel untrusted, especially Marines. HOWEVER, what people feel about it doesn't have anything to do with why it was done. A staffer could have made this PR decision and had an unintended effect of making people feel distrusted, because something was the result of someone's actions doesn't mean it was the intent of their actions. I mean why would the SecDef or a staffer want to make people think they don't trust the Marines?
And why wouldn't they trust them? There's another question that doesn't have an answer? You can "Why would they want to make the Afghans feel better?" and there's a million reasons for that. But you ask why would they want Marines to think they don't trust them, and there's no reason to.