The fantastic YouTube-dl team

  # requirements:
  # sh, sed (BSD ok), tr, openssl, ftp or some other httpclient

  curl=ftp 
  file=${2-1.mp4} # default outfile

  # itag #s are on the wikipedia page for youtube

  case $# in
  [12])
  {
  sed 's/http:/https:/' \
  |while read a;
  do

  b=${a#*://}; 
  c=${b%%/*}; 
  {
  printf "%b" "GET /${a#https://*/} HTTP/1.0\r\n";
  printf "Host: $c\r\n";
  printf "User-Agent: OK, Google.\r\n";
  printf "Connection: close\r\n\r\n";
  } \
  |openssl s_client -ign_eof -connect $c:443 -verify 9 
  done \
  |sed '
  s,http,\
  &,g;
  ' \
  |sed '
  /%3A%2F/!d;
  /videoplayback/!d; 
  s,%3D,=,g;
  s,%3A,:,g;
  s,%2F,/,g;
  s,%3F,?,g;
  s/^M//g;
  ' \
  |sed -e '
  s/&itag=5//;t1
  s/&itag=1[78]//;t1
  s/&itag=22//;t1
  s/&itag=3[4-8]//;t1
  s/&itag=4[3-6]//;t1
  s/&itag=1[346][0-9]//;t1
  ' -e :1 \
  |sed '
  s,%26,\
  ,g;
  s,&,\
  ,g;
  ' \
  |sed 's/%25/%/g' \
  |tr '\012' '&' \
  |sed 's/&$//'; 
  echo 
  } \
  |sed '
  s,%3D,=,g;
  s,%3A,:,g;
  s,%2F,/,g;
  s,%3F,?,g;
  s/^M//g;
  ' \
  |sed '
  s/&https/\
  \
  https/g;' \
  |sed 's/\\u0026.*//' \
  |sed '/itag='"${1-.}"'/!d;'|{ 
  read a;
  exec $curl -4o $file $a;}
  ;;
  *)
  exec echo \
  "usage:   $0 itagno [outfile]
  outfile: $file"
  esac

They do, but it's not like YT can do anything about it except adding some obfuscation and changing that periodically. If it can be viewed on one machine, it can also be recorded. Hence the War on General-Purpose Computing.

I still remember when all you needed to do to download was change "watch?v=" into "get_video?video_id=" in the URL...