Why do Kings count as military units?

I clicked the select all idle military hotkey, my halberdiers pricked their ears and earnestly awaited the orders of their commander.

I right clicked the front line, and they charged fervently towards the enemy.

But faster than the halberdiers was my king, who ingeniously ungarrisoned himself from his castle, and hurtled towards the enemy with such passion he actually arrived there before any of my army.

Cruelly, he was slain by 10 knights.

You have been defeated.

WHY IS THE KING CLASSED AS A MILITARY UNIT.