You are all missing a caveat here. This is an iPhone, which is a GSM phone, meaning it has a SIM card which tells you who the Provider is (not to mention you can punch in, I think #3 or *3 and universally be connected with the provider's support line) so all the guy had to do if he was legit as contact the cellular provider and say "I found this phone, the SIM# is 123456" and they would have arranged for him to deliver it to an AT&T store where they would then return it to it's owner.
Also, as stated he could have turned the phone over the the manager on duty (but for sake of argument, maybe he didn't think of that at the time and/or figured the owner would just sell the phone despite this (which is sadly quite common))
The guy obviously was trying to get something out of apple (either information, $$$, or something else) by trying to force them to admit it was a prototype iPhone4. He was NOT being a "good Samaritan"
Of course it is *possible* that the guy was just an idiot... if he had found a Samsung phone, would he have called Samsung??
The phone bricked itself.