There are plenty of reasons. Many people use these in their emails to get effectively "read receipts". It's not even clear that this was an official military action or just one officer using a browser extension that embeds these images.
There is very very little information you can get from an individual image being loaded by a user agent.
If they're worried about leaks, it seems much more likely that they're interested in who is talking to who, which they can figure out if the email with the tracking pixel gets forwarded.
There is very very little information you can get from an individual image being loaded by a user agent.