I have a feeling this is a built in feature to avoid hundreds and hundreds of photos being stored on the server while not available to the public. When people publicly post, we get an idea if someone is abusing the feature (posting insane amounts of photos each day, which I doubt anyone will do) but if it is done through PMs we have no way to avoid, or even know, when people are using this site to share large amount of photos.
For now I'm going to be leaving this feature as it is, my hope is that if people have pictures to share they are free to share them on the forum. Does anyone have a problem with this? I would love to hear example of why people would need this feature, I could be swayed.