they use adhesion promotor on the plastic before primer so it would stick better. and once the primer sticks the paint will stick to the primer. and so on and so on.
I would think that it would be impractical to paint mud flaps given their flexible nature. No matter how well the paint has been made to stick, the flexing will always cause cracking. Molded splash guards are a different story but I've never seen a set that were pre-painted.
