How does the cop know he's not a felon?
How does he know that cop isn't a police impersonator? How does he know the cop isn't a convicted felon who lied about his criminal record when applying for the job? How does he know this isn't the day a real cop has snapped from the pressure and is in the process of going postal?
If simply being armed in public and these facts not being known is RAS of a crime, then there is also RAS that the cop is a criminal too.
There's a nasty side-effect to that "I don't know you" statement cops love to toss out. The citizen doesn't know the cop either. Impersonating police is a crime, and resisting a criminal's unlawful actions against you is self-defense.