but also informal communications such as private phone calls (within military buildings).

They are worried about iOS being insecure as it's been shown time and time again that iOS devices keep getting hacked and this kind of thing happens. Here is the latest issue with it from 12 days ago.

When dealing with the military, you need to make sure your devices are secure, not "well basic users most likely couldn't break into it."

Now, the title is terrible because it does point out later in the article that many devices will be banned, so it's most likely not just iPhones, but also off the shelf Samsung phones as well. They will most likely want a secure smartphone like a Samsung Tactical Edition smartphone, as those are secure and have military designs.