The Giver reads on many levels, the first one of which depicts difference among people as bad. The utopian society they developed in was wrought out of the frustration people had in their inability to deal with difference. But we find out its not really a utopia, is it? Not everyone agrees? If just one person carries all that emotion and frustration, then its not a whole utopia.
On a deeper level this book plants the seed of an ideology in its readers. The society of The Giver allows no important personal choices to be made, it allows for no weakness of any kind, either. One precipitious plot point is over an infant that is not 'developing' the way it is supposed to. Its not that is has three arms and half a brainstem, its just got a maladjusted temperament. In the story's world, that is enough to terminate it.

Think about this, would this EVER be acceptable? Unfortunately, equally insignificant reasons have been the platform of genocide before. The ideology that different is OK and it is certainly a bad idea to force everyone to be the same is social innoculation. Lowry has given every reader a weapon against falling prey to the gross social disease of not only not accepting those that are different, but doing away with them.