Rocky had no good solution for how to handle the over three thousand prisoners in his hands, so he was open to Berg's suggestion.
People were important, but those who could live peacefully in Thunderhawk City and contribute to its development were the ones Rocky really needed. As for the uncooperative ones, those who wanted to return to their original families and could potentially hinder the development of Thunderhawk City, Rocky did not need them, no matter how many there were. Therefore, he did not mind Berg laundering this batch of prisoners of war.
Although it would cost a whole thousand people in the exchange, as long as the rest remained compliant, the price was worth it. Moreover, Thunderhawk City's capacity could only accommodate five thousand people, while the prisoners plus the original inhabitants already exceeded six thousand. Reducing the number by a thousand was just right.