Quote Originally Posted by Vahnavoi View Post
Monarchy, feodalism and religion are systems. You don't bow to the king on the top of the pole just because he's individually a swell guy - you do so because it affirms the position and concept of authority, an important thing to anyone who'd aspire to be a figure of authority themselves. Remember: organized groups over individuals. It's about the crown and what it represents, not just the guy wearing it.

If you wanted an alternative twist from this, the devils could switch sides to the former king's daughter, provided she can show her claim to be more legitimate than the usurper, and indeed this may have been part of the long con from the start. Of course, it would also mean that by helping the princess, the player characters end up helping the devils further down the line.
It's also possible a rival of the Devil who set up the usurper has set plans for the princess to triumph, or will jump on the winning horse if she can demonstrate she has what it takes to be said horse.

Alternatively the PCs could try challenging the usurper's pact in a Devilish court, or try to go above the bad guys' head and talk to the Devil's superior.