They literally say, it’s his atonement. We still don’t know much about his past but something happened for them to be treating him like this (wooden horse). We know it’s stupid how this kind of behavior could be considered atonement but mc genuinely looks like he’s been brainwashed into thinking this is the right way to atone. There’s also religion that plays a role on his character. (All the talk about god in the start.)
But I do agree that it’s a hard read.

Such a hard read, I don't get why the uke is just accepting being treated like that when he is a literal knights, someone who's capable to fight.
I don't even get his behaviour is general maybe it's psychology thing. I only saw that it's by the same author as legs that won't walk after reading then everything make sense.