Why does ksh exec() the last command in a shell script?

Issue

• After running the last command in a script the parent process dies. In the following example child1.sh executes /bin/sleep.

• The ps output shows the sleep, however the script that is calling the sleep (in this case child1.sh) doesn't show :

  [root@dhcp ~]# cat kshtest.sh 
  #!/usr/bin/ksh
  #
  echo "Starting Parent, DATE: `date`"
  echo "Parent PID:$$"
  echo "Executing child process: ./child1.sh &"
  ./child1.sh &
  echo "Child PID: $!"
  /bin/sleep 120
  echo "Parent Finished, DATE: `date`"
  
  [root@dhcp ~]# cat child1.sh 
  #!/usr/bin/ksh
  #
  /bin/sleep 200
  
  
  LINUX @Runtime:
  ---------------
  $ ./kshtest.sh
  Starting Parent, DATE: Mon Oct 25 16:54:00 CDT 2010
  Parent PID:11477
  Executing child process: ./child1.sh &
  Child PID: 11479
  
  PS capture:
  root  5478  5477  0 15:12 pts/2    00:00:00          -bash
  root 11477  5478  0 16:54 pts/2    00:00:00          /usr/bin/ksh ./kshtest.sh
  root 11479 11477  0 16:54 pts/2    00:00:00          /bin/sleep 200                    <----- There is no child1.sh process
  

• This behaviour is not seen in other shells.