I think from a purely legal perspective if a landowner gives you permission to metal detect, hunt deer, look for arrowheads/fossils, or pick blackberries on his property once he gives permission without stipulations he has waived his rights to anything you find/kill/pick. Any MDing lawyers out that want to weigh in on this? Of course legal rights may vary depending on where you live.
Like LittleJohn and others I too have shared my finds with the landowner. I had a case years ago where a lady gave us permission to detect on her farm. We found one Indian head penny, and nothing else. We made the mistake of showing it to her. She would never allow us to come back because she was afraid we would find all the buried jars of gold and silver on her land (which probably didn't exist to begin with).