If Alistair remains, he will be used to provide magic to that territory's barrier. If he returns to the kingdom, he will be forced to do the same. What's the difference? Perhaps the fact the kingdom has more people and an established system. The territory, however, has few people but they would have a whole lot of greed and ambition with their own barrier. Plus if two of the other people pumping magic into the crystal are royal princes, you can bet they won't be squeezed dry. Alistair, however, if nobody in their eyes.
Alistair asida, Leila obviously shouldn't contribute to the territory. She was bloody kidnapped and brought there against her will. That prince demonstrates his psycho qualities by having the audacity to suggest she should help the territory. That is, unless they pay her a king's ransom. Then, why not. If the territory is planning to start to blackmail the kingdom with the magic crystal supply, geopolitics are changing anyway. The downside is that she wouldn't have a clue about what the kingdom's goals are. If she helps the territory too much, even as a business transaction, the kingdom might judge her a traitor.