programmatically added SliderJoint2D not respected

How can I make sprites fall straight down, and resist any sideways motion whether by dragging or physics?

My code instantiates a prefab at runtime which has an attached script.

The script runs the following function after a while:

 void addSliderJoint() {
     myVerticalSlider = gameObject.AddComponent<SliderJoint2D> () as SliderJoint2D;
     myVerticalSlider.angle = 90;
 }

After this functions runs, I'd like the sprites to only move vertically, but they seem happy to move without restriction. I can "see" the SliderJoint2D in the inspector during runtime, and if I change its settings in the GUI, the sprites suddenly respond to it.

Sample repo available at https://github.com/thunderrabbit/finna-be-octo-wallhack