Why do the Spaniards kidnap Bright Morning in Sing Down the Moon?

Bright Morning and Running Bird are kidnapped by Spanish slave traders. These men look for young Indian girls to sell in the city to families who want domestic help. The slave trade is apparently a lucrative occupation, and essentially, Bright Morning and Running Bird are kidnapped to make money for their captors. Bright Morning says,

For many years now (the Spanish slavers) had come to the Navaho country and stolen girls to sell to families in the town who needed girls to cook for them and to wash their floors. One of the Navaho girls had escaped and come back to Canyon de Chelly and told us what had happened to her. (Chapter 4)

The slave traders prey upon the young Navaho girls as they watch their sheep in the grazing areas. In this way, they hope to be able to take their captives surreptitiously, without having to confront the warriors who would defend them. When Bright Morning and Running Bird are stolen, the warriors in their tribe are away on a raid, but the slavers do not know that. They are very careful in making their escape with the girls, often stopping to listen for the sound of following hoofbeats (Chapter 5).