Let me start from the end of your comment, your example. Frankly, aside from her family and friends, nobody cares if you don't help her. Oh sure, people say you're an asshole, and you should help her, take her to the hospital, etc. But tell them to do it themselves, and they make up an excuse and hurry on: Bystander Effect. On the other hand, raping her is a crime unto itself, punishable by law. That's where your analogy goes astray, in that you attempt to substitute a clearly illegal act for a legal, if unethical one.
Which leads me on to the first part of your comment. Given that the wallet was lost, it left possession of its owner. According to Michael v First Chicago Corp. Illinois, 1985, "A finder of property acquires no rights in mislaid property, is entitled to possession of lost property against everyone except the true owner, and is entitled to keep abandoned property.". Meaning unless the original owner tracks you down (which, let's face it, is quite unlikely in any moderately sized city, let alone a metropolis) and reclaims his wallet, it is, indeed, yours to keep. Whether it was ethical to keep or not.
The fact that there was contact information inside is quite irrelevant, given that the true owner has to claim the wallet from you to enforce his possession, in which case you must yield it. Sure, people might think even less of you for not even attempting to return it, but that doesn't make it any less legal.