It ties into the ancient samurai tradition of practicing for battle by first beheading a bunch of criminals. Beheading prisoners is to war as the driving range is to the golf course.
For example, the 60-something retired samurai Yamamoto Tsunetomo said in Hagakure: The Book of the Samurai, from the early 17th Century:
Yamamaoto Kichizaemon was ordered b his father Jin'emon to cut down a dog at the age of five, and at the age of fifteen he was made to execute a criminal. Everybody, by the time they were fourteen or fifteen, was ordered to do a beheading without fail.
Last year I went to the Kase Execution Grounds to try my hand at beheading and I found it to be an extremely good feeling. To think that it is unnerving is a symptom of cowardice.