If they wanted B, they should book B. It's not up to them do decide which seat is best for the person in B. Maybe the person in B doesn't want to get cold by the window, or doesn't want to sit in the aisle because they don't want to be hit by falling luggage, or the cart, or someone walking down the aisle. It's not my choice, but I am not going to question someone wanting to sit in the middle. I was on a 763 were some guy kept complaining because a guy didn't want to move out of a bulkhead seat in First (transcon). They person in the seat said he always sat there and booked month in advance. This didn't stop the other guy from making comments for most of the flight. He even whine about how normal people didn't like bulkhead. Never mind that the first row of F on a 763 has the same comfort as other rows. If I want to stay put, that should be the end of it.