Why did scientists justify the cuckoo despite the fact that cuckoo foundlings throw other chicks out of the nest?

There is no need to justify animal behavior. Also, you do not need to look for morality in the behavior of animals and divide their actions into good and bad. From a biological point of view, there are no such concepts. All animals strive for reproduction, survival and preservation of offspring. The cuckoo throws up its cubs so that they can survive. The bird itself cannot feed them. In this way, it retains its appearance. Cubs of cuckoos discard other birds also for their survival, as they grow and need more food.