If a utopian society is perfect, why does the book The Giver come up when I search for a book about a utopian society?

The Giver comes up under the category of "utopian novel" because it is a novel that is concerned with the concept of a utopian society. While the narrative is definitively about a dystopian society, it certainly points to the challenges of attempts to create a utopia, a worthy examination, to be sure.

The society of Jonas, who is the main character, is purportedly a perfect society: the environment is controlled, everyone plays a productive role...

The Giver comes up under the category of "utopian novel" because it is a novel that is concerned with the concept of a utopian society. While the narrative is definitively about a dystopian society, it certainly points to the challenges of attempts to create a utopia, a worthy examination, to be sure.

The society of Jonas, who is the main character, is purportedly a perfect society: the environment is controlled, everyone plays a productive role in the community, desires and sexual urges that could go awry are controlled, emotions are controlled as euphemistic words are used and people are taught to be sensitive to others' feelings. Each age group is treated similarly, so that there is no competition among the children. Finally, at the end of their twelfth year, the adolescents are assigned their roles in life. Only one is appointed as the Receiver of Memory for the community. Many of these memories are painful, but one person must retain them because they may be necessary in times of crisis.

Sameness is considered the solution to conflict, but it is not ideal as there are too many adjustments, some of which are inhumane, that must be made in order to have the controlled, perfect society. Jonas is appointed as the Receiver, and his world changes for him because he learns how these controls really work. In Chapter 12 the Giver tells Jonas, 

We relinquished color when we relinquished sunshine and did away with differences....We gained control of many things. But we had to let go of others."
"We shouldn't have!" Jonas said fiercely.

There is no question that at first the society of Jonas seems a peaceful, contented world. However, as the narrative progresses, this utopian society reveals flaws, and it becomes apparent that the perfect society is not so perfect. But, at one point it is almost perfect; at one point it is a utopia.