The Knights running the punishment hall were once soldiers under the Count.
They were all hard-headed and valiant men.
When they found out that Noah had schemed to kill Roy for no reason, they were disgusted.
But their disgust turned to anger against Noah when the fact that Roy was coerced into a duel of honor because his maids and friends were threatened was revealed.
That was playing dirty, something a noble, especially a mage, should never do.
Targeting someone's family was considered taboo. It was also the most dishonorable act known to knights and soldiers. It was called "playing dirty like a devil."
They wanted to slap some sense into Noah right there and then.
However, they couldn't kill or harm him as he was a Baldwin and the son of their Lord.
Thus, they put him in a place where he would experience the life of a prisoner.
Noah's punishment was to stay in a tiny, damp room, eat the food of a prisoner, and reflect on his actions for six months.