Wow, I think you are right. Characters get 360 LoS, monsters may turn in any direction without penalty according to the rules under monsters so I assumed that if they can turn without penalty, they can shoot without penalty. But it is true that in the shooting rules, where it says modifiers, under moving and shooting 'even a simple turn can effect a models concentration and reduce his to hit roll'. I couldn't find anything saying a monstrous mount may turn without shooting penalty, I always thought they could and my opponents played as such but it seems they can't.