When you shoot at a character riding a monster the usual restrictions for shooting at characters apply as explained the the section on Heroes and Wizards. However, monsters are so big that the chance of being able to find a unit of comparably sized creatures to shelter inside or nearby is rather slim. A character riding a big monster is therefore easy to pick out as a target. Because the monster is a large target the enemy adds +1 to his to hit score.

The normal -1 to hit that applies to man-sized characters on foot or as cavalry does not apply to characters who are riding monsters. The whole target is simply so obvious that there is no doubt as to where the character is! You might wish to re-read the rules for shooting at characters in the Heroes and Wizards section to clarify this.