It appears that iMovie is so CPU/disk intensive that Apple wants to discourage you from doing anything else while you are working with iMovie. It may be annoying, but I doubt it was done without a good reason. I have just grown to accept it and assume my Mac is not a computer when running iMovie...it is a Digital Video Capturing/Editing device. I trust the Apple Engineers did this for a good reason and I haven't attempted to second guess them.