My assumption is the person put a shackle so that the cat might get tangled up while in the process of scratching.
The fact that the cat is very chill about it is... interesting. My two cats would absolutely panic and very likely hurt themselves if they were in this boat. Yet this one is just blep.
I've heard with dogs if you tie a chicken they killed to them for a while they learn to associate killing chickens as a bad thing as they don't like having something tied to them. Something similar might be going on here to teach the cat scratching stuff is bad.