The point is whether the process is considered animal torture or not, regardless the type of animal. I don't think anybody would object to caponizing if it is done with anesthesia, but then it will not be practical nor economically beneficial for the people or the industry to caponize.
The same reason why western world is criticizing the halal/kosher slaughter which basically let the meat animal bleed to death in pain. If you look at it from the perspective that they are being killed anyway, what is the point arguing on a little bit of extra suffering. Or how we look down on the japanese restaurants that serve a live fish so the costumers can pick the flesh when they are at the freshest stage while the fish is gaping for air on the sizzling plate. All for the most tender and better tasted flesh for the satisfaction of people who eat the fish.
It may be an extreme example but it makes it easier to see the pattern.
They are now also working on banning battery chicken, but i guess it would be an uphill battle.
To stay on topic, this was my rooster Barney, a cross between silkie and barnevelder.