Do Mutalisk attacks bounce from air to ground and vise versa?

Example: Mutalisk attacks Viking, the attack bounces to a marine.

If so;

Do you feel this is fair?

Yes, Mutalisk attacks bounce from Air to Ground units. This is what made a difference in some ZvZ SC:BW games for example, where you would take your Zerglings and Overlords with you to Mutalisk vs Mutalisk fights to reduce the bounce attack a little bit and gain an advantage.

It is also part of the gameplay and adds more options for the players also making the Mutalisk more versatile.