IANL. Under American law, if the owner doesn't enforce exclusion the land can become public through implied dedication if the public continues to use it over time.
Implied dedication doesn't make the land public, it simply creates a public easement allowing the public to continue using the land in the manner it was being used that created the implied dedication.
For example, if people openly hike through part of a private property for 5 years (in CA), and the owner knows this and does nothing, then after 5 years there will be a public easement for the public to continue using that part of the land as a hiking trail and the owner can no longer prevent that. OTOH, if he puts up signs on the 12th month of the 4th year, saying "Hiking permitted by owner", then no public easement is created, and the owner can subsequently close off public access at any point.
