Because it wasn't "stolen". Of course Apple would say it was, especially if they plan on setting up any future litigation. However proving that it was stolen would be on Apple, and if the guy who found it can prove that he attempted to call Apple (which would be easy with phone records and his customer service ticket) their case is weak as hell.
In either case, the fact that he knew it was Apple property, and then procededed to sell the stolen/lost hardware in exhange for money violates a state law, at minimum. I think whether there is further litigation really depends on whether or not Apple decides it's worth it.
To say that he can clear himself by saying/proving he called Apple is like saying that if you leave your car unlocked in a bad neighborhood and before someone steals it, they try calling you first to tell you you left your car unlocked. Then, when the call goes to your voicemail, they proceed to sell your car to someone else. As far as I know, that doesn't make it ok.