There is nothing fair or just about the "reaping" that takes place in the book The Hunger Games. There are three main reasons for this.
First, Panem, the capitol city, is unjust. Instead of protecting, serving and benefiting the districts, Panem exploits the districts. One of the main ways they do this is by creating a system called reaping. In fact, the reaping with other forms of exploitations create a perpetual form of slavery. Keep in mind that no one from the capitol city undergoes a reaping. This is an important point to keep in mind.
Second, the people are so poor that the book says that people actually volunteer for the reaping. Here is a lengthy quote, but it shows the poverty of the people that they would risk their lives just to survive.
You become eligible for the reaping the day you turn twelve. That year, your name is entered once. At thirteen, twice. And so on and so on until you reach the age of eighteen, the final year of eligibility, when your name goes into the pool seven times. That’s true for every citizen in all twelve districts in the entire country of Panem. But here’s the catch. Say you are poor and starving as we were. You can opt to add your name more times in exchange for tesserae. Each tessera is worth a meager year’s supply of grain and oil for one person. You may do this for each of your family members as well. So, at the age of twelve, I had my name entered four times. Once, because I had to, and three times for tesserae for grain and oil for myself, Prim, and my mother. In fact, every year I have needed to do this. And the entries are cumulative. So now, at the age of sixteen, my name will be in the reaping twenty times.
Finally, the reaping aims at exploiting children. Notice the quote says that you are eligible to be reaped at the age of twelve. This means that the reaping systematically harms poor children. This does something to the psyche of the people. In conclusion, no matter how you look at it, there is nothing just about the process of reaping.