this region of the world, on the north american continent, commonly referred to as the "united states" has a massive crime problem. large areas are occupied and restricted by criminal organizations. of course i'm referring to governments occupying "public land".
a friend and i like to go out to a local public school and play with an aerobie (greatest flying toy ever!). we've never had any problems with the gubment there, but we used to have problems when we played at a nearby park. they'd close the park from time to time for no apparent reason. as you might expect, we played there anyway. inevitably, some low-level bureaucrat would appear to shoo us away. if these guys were of any level of wit, i'd have loved to ask them from whence they derive their authority.
authority over property can be derived only from the property owner or his agent. now, the guy who informed us of our unwelcomeness was an agent of the the city, but does the city have authority over property?
you may have heard the misguided phrase "ownership is control". libertarians reject this. ownership is, in fact, not necessarily control, nor is control, ownership. the city controls the park in the same way that a car thief controls a vehicle - through force. a car thief can't own a car he's stolen because he didn't come into control of it through legitimate, or voluntary means. ownership can never be the product of involuntary interaction, whether it be theft, murder, extortion or occupation. similarly, the city, or any other government, can not own anything because it gains all that it has through direct or indirect force or the threat thereof. it can only hope to control property, not own it.
governments do not own roads, parks, buildings or anything else. they only control them. they secure funding for, build and maintain these things by force alone, often forcibly excluding competition in the process. people will often make the argument that if you don't believe that government should exist and that how governments obtain property is wrong, then don't use the roads, libraries, etc. i solemnly swear i will never use anything that the government owns (which is nothing). the roads i drive on belong to no one and i feel free to use unowned property as i wish.
control is not ownership. hopefully, owners control their property, but governments also control property without owning it. it's important to understand the difference.