You've probably seen old movies where the protagonist is approached by a Nazi or Soviet guard and ordered to "show your papers." We know that's a tell-tale sign of a police state. So if police ever ask you to show ID during your travels, it's natural to feel violated.
In a free society, citizens who are minding their own business are not obligated to "show their papers" to police. In fact, in the United States there's no law requiring citizens to carry identification of any kind.
So when can police ask for ID?
Carrying an ID is generally required if you're driving a vehicle or a passenger on a commercial airline. These requirements have been upheld on the slippery premise that individuals who prefer not to carry ID can choose not to drive or fly.