I knew some corporations who had strong federal presence, more
  or less permanent government contracts [and the companies you
  mention are absolutely huge] did this, but I was thinking that
  most prison labor consists of road cleanup and "community
  service" type labor. But I hadn't considered that they are
  utilized in a "keep the money in the USA" way. [make use of
  Americans instead of foreigners" thing]. It is rather weird /
  disturbing to consider.