The goal of the lander is not primarily to spread ashes. As for being a place sacred to the Navajo, there are no other Moons for them to claim to be sacred. It is one Moon, shared by all of Humanity, the Navajo alone do not get to lay claim to what should be done on it. Especially since every other culture has some sort of spiritual belief about the Moon and they don't typically hold the same expectation of having everyone else obey their spiritual beliefs.