Case law varies quite a bit from state to state as to what constitutes a public place. Generally, the rules are that it is a place where privacy cannot be assumed, the general public is freely able to visit and is not private property.
However, as a photojournalist, I avoid photographing people who don't want to be photographed unless there is an overriding news reason to photograph them. For example, a random voter who asks not to be photographed won't end up in the paper while a known party activist who is trying to keep somthing hidden will be photographed if that thing is newsworthy.
Thank you for the clear explanation.