Desmomoto
On the track you have no need for mirrors. On a bike you are normally required to remove them as if you come off the first thing to happen once the bike goes down on it's side is the mirror shatters and covers the track in shards of glass. Cars don't fall over on their side during a crash, so mirrors not a danger. Also car mirrors are very difficult to remove as they are usually electrically controlled etc etc.
I also know they like you to remove your mirrors to stop you looking in them. On a track it's all about whats in front, not behind.