An advanced society of intellectually superior aliens, could have conceivably advanced morally also. Their view might well be that "Who are we to assume that our culture has so much to offer that we should modify destroy the existing culture of the earth, particularly if they are happy as they are?" I have observed this attitude by Anthropologists in regard to primitive cultures.
They believe that there are technically advanced and primitive cultures, but there are only different types of cultures, not different degrees...and they are very critical of missionaries, who have reputedly done more harm to the primitives than good.