Oh man...... i wonder if the prince knows about this
Likely not.
Given how Rachel described the king, this was probably a choice he made on his own. It's a political and dynastic decision and from a monarchs point of view it normally won't matter what the crown prince thinks. He has to obey, the king's decree should be absolute. But we don't know how the royal family's internal power balance works. The king wants to secure the power of the Saintess for the royal family, but the crown prince might still have his own agenda, especially if he loves Rachel.
Additinally, Rachel described His Majesty as a man who values justice, loyalty and merits - the king shouldn't be some unreasonable tyrant, then. He will probably compensate the Marcels handsomely. That's what I would do, or try to do... I mean, he betrays a trustworthy and loyal follower. Even a king cannot afford to do that lightly because loyalty and trust mean everything in such societies. Fealty binds both sides, not just the subject but the ruler as well. Betraying someone who swore fealty would undermine his rule, slowly but surely.
Let's see how this develops. The Crown Prince will probably put a stop to this.