I'm sure the military has always hidden information from civilians. Some of it is necessary to ensure public and troop safety, but other times, it's just withholding controversial information. At the time, had civilians known there were American POW's killed in Hiroshima, I'm not sure what the result would have been, but the military leaders and the President had already decided on a course of action.
In the 1970s, Americans learned that at least a dozen United States prisoners of war were killed by the atomic bomb in Hiroshima. This comes as no surprise as the military often hide controversial information from the public. Imagine if the public knew everything, there would be chaos. On the other hand, this is wnhy there are so many conspiracy theories out there.
I did not know that some United States prisoners of war were killed by the atomic bomb in Hiroshima. But, unfortunately, that doesn't really surprise me. The military often hides this type of information from the public. I believe the reason they do this is not be deceitful, but to control reactions from the public. While the U.S. celebrated its victory over Japan with the bombings of Nagasaki and Hiroshima, it would have been ill-advised for the military to let the general population know, at that time, of the deaths of American soldiers from an American bomb. Such knowledge could have triggered protests, and possibly riots, throughout America, undermining the necessary act of bombing these cities to stop Japan and end WWII.
The answer to this question is likely somewhere in between. In most cases, the military (and the government as a whole) does not necessarily hide information from the general public, but certainly does not go out of its way to broadcast information that Americans might find distasteful. American intelligence agencies, on the other hand, do tend to omit important information for years after the fact. This is all typically done in the name of national security. Most of the information protected by this claim is likely valid and must be kept from the public eye to protect operations and individuals, but some could likely be shared.