When you are fighting house-to-house against an enemy who is not in uniform, so that it is difficult to identify him anyway, the LAST thing you need is some civilian "observer" getting in the line of fire.
In that kind of situation you have to assume that anyone not wearing a uniform is an enemy bent on killing you and you don't have time to ask for someones press card.
While it certainly can give rise to all kinds of suspicions, the real reasons are really pretty clear. It's simply too damn dangerous to the men doing the fighting.
I feel confident that any combat veterans on the forum will back me up on this. Journalists traveling with you on an operation is one thing; journalists wandering around loose in a half-destroyed city are something else entirely.

troll