House Tyrell of Highgarden is one of the Great Houses of the Seven Kingdoms, overlords over the Reach. A large, wealthy house, its wealth is only surpassed among the Great Houses by House Lannister, and the Tyrells can field the greatest armies. Additionally, if they call the fleets of their bannermen the Redwynes, the lords of the Shield Islands, and the coastal lords, they can command a navy that equals if not surpasses the royal fleet.
People everywhere have their differences. In some places, the highborn frown upon those of low birth. In other places, the rape and murder of women and children is considered distasteful. What a fortunate thing for you, former Queen Regent, that your daughter Myrcella has been sent to live in the latter sort of place.