Just because a set of documents are leaked, exposed, stolen, published by someone or handed over to people who are not authorized to view it, it does not automatically reset their classification label.

Now you might say "so what this is dumb". And it is and it has nothing to do with you unless you have a clearance. People with a clearance have signed contracts and other documents that say "there are penalties involved if you commit a security violation". One such security violation is "copying or accessing classified information on unclassified systems". That's it. You see where I am going hopefully.

You are a grunt on some army based working with crypto radios. You have a clearance. You hope to work for the CIA or NSA maybe when you go back to civilian life. News about leaks comes out. You browse HN or Reddit at work during lunch. See news about leaks. Click and oops! you have just committed a serious security violation. You are accessing classified information on an unclassified system. This _could_ get you into trouble. If anything at least when you are polygraphed if you apply to work at the 3 letter agencies later.

So think of this filtering as a "courtesy" to help them inadvertently break some serious rules.

Now, does it sound silly and pedantic? Yes. Do I personally agree with this interpretation? No. But that is how it is. And I think this is the reason for filtering not that it tries to prevent oh I don't know an armed rebellion.