Actually, the weapon stats are added to the weapon tab once you add the shield. That way, you can equip the shield when you want to get the shield AC, equip the weapon when you want to shield bash, or both if you have the Improved Shield Bashing feat.
Since the masterwork/enchantment bonuses as a weapon are separate from the masterwork/enchantment bonuses as a shield, all the shields show up in the custom weapons section, with an edit button that allows you to go in and edit the weapon enchantments after you've added the shield.
Since the masterwork/enchantment bonuses as a weapon are separate from the masterwork/enchantment bonuses as a shield, all the shields show up in the custom weapons section, with an edit button that allows you to go in and edit the weapon enchantments after you've added the shield.