Yes, it was unusual and cruel in a way. For tall people who didn't want to be part of his army he organized kidnapping. He even marked children who had the potential of becoming tall when they grew up. It is strange that no qualifications, other than being unusually tall, were required to join the Prussian army.
Yes, I think that Prussia's King Frederick was a little insane because of his obsession with creating tall soldiers. This was not just something that he wanted. Instead, it was something that he took drastic steps to achieve, even at the expense of his loyal soldiers' health and comfort. It was not right!
From all over Prussia he had his agents look for- and oftentimes kidnap- men suffering from gigantism. In striving to create his own personal soldier core of giants, the king instructed his subjects to immediately signal the authorities whenever they should become aware of exceptionally tall men in the vicinity. He also made clear to his political allies that they could keep their gifts of gold for themselves as long as they provided him now and then with fresh giants to fill up his stock.
Yes, his obsession with creating tall soldiers was cruel and unusual because it meant many people were bread, not for their health, but for his benefit. He did not care about them as human beings, he simply wanted a better military for his own gain. It was an inhumane view on society.