Why is watching release forbidden to children, but not to Jonas

Jonas is allowed to watch the release of a child because he is the Receiver.
In the society in which Jonas lives, pain and suffering have been eliminated. Because people are not permitted to be in situations in which there is suffering, they are not made aware of what "release" actually entails. In chapter nineteen, Jonas mentions to the Giver that his father is going to release one of the identical twins recently born because such twins are not allowed in his society. "I wish I could watch," Jonas adds after he tells the Giver that his father, whom he considers "such a gentle man," performs the ceremony and makes the little twin "clean and comfy."
The Giver explains that Jonas can watch because he is allowed to "ask anyone anything." Since the ceremony has already been performed, Jonas decides that he will view the tape which the Giver tells him is available. The Giver has Jonas himself ask for the tape on the speaker. The video screen then comes on, and Jonas is surprised to see a small room because he believes that release involves a ceremony. As the video continues, Jonas is shocked when he witnesses his father inject a needle into the smaller twin's head. Jonas sees the baby die as his father calmly speaks in his "special voice" for babies.

He killed it! My father killed it! Jonas said to himself, stunned at what he was realizing. He continued to stare at the screen numbly. (Ch. 19)

After a short time, Jonas feels a "terrible pain" as he is made aware of one of the harsh realities of his society.