The hair from your body (legs and arms) grows at a much slower rate then your head. However if you are a drug user the toxins will stay in your body hair for the same time frame as your head and how many of us cut the hair on our body as often as the hair on your head. So by taking the hair from your body they get a better history of your drug use then if they took it from your head.
If you dont do drugs then you have nothing to worry about. Right?
Guns don't kill people!
Bulletts don't kill people!
Holes in people kills people!