When we all enrolled for SkyMiles (or predecessor programs), we agreed to follow the rules. We agreed Delta could change the rules. We agreed Delta would be the sole arbiter of the rules. The rules say travel by one human passenger yields mileage for one account. No miles for second seats. No miles for cabin pets. No miles for flying violas. There is no basis for trying to rationalize the rules any more than trying to figure out why one-way tickets can cost more than round trips.