Public property is government owned property that is dedicated to public use.  Examples are national, state and city parks, public schools and colleges,  historic sites, bike trails, conservation land, wildlife sanctuaries, national  seashores, courts, libraries, etc. Its still owned by the local, state, or federal gov't. So... even though they are peacefully protesting, they can still be trespassing.  Im not saying i agree with it but thats the fact of the matter