The gemara discusses someone moving into someon'e unused chatzar and cites it as a possibility of ze nehene v'ze lo chosayr - this one gets benefit and this one loses nothing.
Didn't Chazal have the concepts of the sanctity of private property and trespassing as we do? How about privacy? How about the dinim of neighbors? To me the concept of someone moving into my backyard without permission even if I'm not using it seems a little outrageous.
Sam Kosofsky
As much as we should respect the "sanctity of private property," we must certainly respect the sanctity of Chazal. We certainly have no cultural superiority over Chazal, Chas v'Shalom. As for respecting a person's privacy, Chazal even inhibited the use of one's own property when it causes an infringement on another's rights to privacy (see the first two Perakim of Maseches Bava Basra).
Obviously the Chatzer under discussion in our Gemara is not a backyard which would infringe on the owner's rights to privacy. That would not be "Zeh Eino Chaser." In addition, if the owner protests in advance he can stop another person from using his property even if the owner is not "Chaser" (according to most Rishonim).
If the property is not in use and using it will not hinder the owner, not only do Chazal exempt the user from payment (Zeh Neheneh v'Zeh Lo Chaser), they deem it unethical for the owner to forbid its use ("Midas Sedom").
D. Zupnik