It is because Lenina has never been or seen that situation. Lenina comes with a different cultural background from the people of New Mexico. Lenina was very close-minded and believes that she has the upper-hand in the given situation.
The motivation behind why Lenina so startled by the old man and the ladies nursing infants is that she has never observed anybody nurture a youngster. She finds the regular route in which the Indians live to be appalling. I hope this helps.
