When you audition an actor, you hardly ever just have them read one thing. You want to see the range of what he can do. Odds are they just had him red for Bo to see how he handled Bo's material, as well as Luke's. According to most people's recollections, John was essentially already getting the part of Bo, so long as casting could find someone to play off of him. They probably never had Tom in mind for the part of Bo, but they did want to see how he'd handle being the more rash one.