@Twenty4carats : He asked to be punished, probably feeling guilty conscience that he failed to convince the girl to take things seriously that it result in all this.
He probably was very sweet to her initially, then when things got serious (i.e. when Ill got hurt) he finally woke up and tried to get her to realize she need to do her job.
She refused, and once she started throwing tantrum (that first wave of magic) he decided to stab her because if left alone she'll tantrum the country to ruin.
Her final magic wave got blocked by Tsukina but the prince still asked to be punished for his failures anyway (he probably viewed her tantrum as partly his fault even)
But everyone probably thought he's too harsh on himself + with Tsukina's magic basically negating any potential damage he can use to justify any serious punishment, they just left him to house arrest.
@Superkrza : I can see a new savior hearing polygamy being legal and try to go full reverse harem about it, obviously clashing with Tsukina when the new kid try to grab Ill into her own harem. If Ill avoid outright refusing then that'd be a potential rift between them.