It's obscene that a defender riding out to attack forces the besieging faction to assault the walls.
The basic conceit of a siege is that the attacker doesn't want to do that yet. It can easily force a loss when you wouldn't have committed your current forces to an assault on the walls.
It doesn't make any sense that it is the defender that gets to force that to happen, when they want it to happen. If they want to break the siege, they need to come outside.