If you're on a military facility, even on a road through the facility, you're SUBJECT to a stop, but it won't always happen.
I said "military facility" and I'm 100% correct.
They CAN stop you, but they don't 99.9% of the time.
I can't comment on federal landmarks and national parks, though. But, I know the military itself cannot establish roadblocks on other federal property, even accessways, without specific authorization, as this would be an law enforcement responsibility.
By your logic, the military can set up roadblacks on many US and state highways that pass through federally owned areas in the Western US.
If they took this to the extreme (consider BLM and Forest Service land, etc.) it would be like living under martial law. You think that National Park rangers should have the right to stop vehicles without a lawful pretext, too?
What is so friggin' special about federal employees and the property on which they earn their paychecks? Last I looked, it was THE PEOPLE'S property.""
The state I live in is 88% Government property or more.