Here is a simple explanation as to why a Jew can benefit from a mistake made by a non-Jew, or keep an item lost by a non-Jew:
Common practice is "finders keepers, losers weepers." When you lose something, or when you post a pricing error, etc., you don't expect anyone to return it or correct the mistake. That is accepted international practice, and that is what is mandated by Jewish law.
However... there is a club, and one of the perks of the club is that club members return lost items to each other, even though it is not otherwise required. Club members also correct pricing mistakes of other club members, and do not lend with interest to fellow club members. All these practices are not mandated by basic international etiquette, but just as any fraternity has certain added loyalties, so too club members have certain loyalties that are NOT REQUIRED by standard morals.
Who can join the club? ANYONE! But club rules require keeping the 613 bylaws. You do not need to be born into the club to get the perks, but you must undertake to follow all the rules. If you are willing, you will also get all the club's perks. Chose not to? That's okay. You will not be treated any less. But you are simply not eligible for fraternity loyalty perks.