When I was restoring my old Manta:
I had the same thought with the Corvair tranny. I was working on it and could spin the rear wheels. I never noticed that the other side spun in the opposite direction. I nearly went nuts. I kept thinking what was going to stop this thing from rolling? Its in gear and the wheels are turning! Its not supposed to be able to roll in gear!
I posted and posted and had people telling me all these major things that might be wrong with it. Finally someone said "its not going to roll anywhere...the wheels are spinning opposite...its normal".
Whew! I almost tore into that thing for NOTHING!