If a cop sees someone with what appears to be a gun, or shaped like a gun, he must assume danger.
Yet a citizen should not assume the same when they see a cop with a gun?
No, a police officer and anyone else for that matter is obliged to look at the overall context of the situation and determine if they or anyone else are in danger. And more precisely in most states if they are in imminent danger.
By your logic a police officer would be justified in assuming "danger" if he walked into the local gun store and saw the owner and a customer handling a firearm.
Just because you come across someone carrying a firearm doesn't mean you are in danger. Why around here we see people with firearms frequently. Sometimes they are even moving furtively through the woods in camouflage outfits. And sometimes you see them with a dead deer or turkey too.