This patch removes the <Capturable> stat from all siege engines. In reality, war machines are not more capturable than arrows, swords, chainmails, horses, or elephants, none of which are capturable in 0 A.D. Rams, ships, and other military units are currently not capturable either.
More than anything else, artillery requires specialists to operate them. Artillery being captured, turned around in the heat of battle, and starting firing on the troops of its previous owners is rather unrealistic.
Moreover, it can be rather annoying for players who want to destroy enemy artillery that capturing is the default.
[EDIT] In compensation, artillery armour is severely reduced, making them much easier to destroy by e.g. spearmen. Battering ram and siege tower armour is not changed.