You know that's the classic example of what people say when they're incorrectly believing an urban myth? "I read it in a newspaper, I don't remember when." Humans aren't very good at recalling where they read or heard something years ago, and Snopes is full of failed attempts to find these sorts of stories in the newspapers where people claimed they found them. I know that I have fallen prey to this trick of memory myself, I think most people have.

It's also quite possible that this is a misreporting of what actually happened. No question that setting booby traps is illegal and you can be liable to anyone injured by one even if they're trespassing. The idea that a trapper would leave a bunch out on the floor for oiling purposes sounds absurdly far-fetched to me, and the jury may have decided that it was more likely than not the trapper was lying about why he laid them out. Civil cases are decided by a preponderance of the evidence so that's all it would take.