Many people don't understand the distinction between client-side and server-side. Javascript typically runs on your computer. I know that it seems to be out there on the Web, but it's not. It's running locally on your computer. It's using your CPU and your memory. It's important that normal people understand this.

Edit: We disable Javascript because we don't want other people running code on our computers without our consent or knowledge while we browse the Web.

Edit2: Here is some Javascript to peg a client's CPU (this can be malicious or accidental, but either way it's why I run Javascript blockers):

  <script type="text/javascript">

  function burn_cpu(y)
  {
      var x = 0;

      while ( x != y )
      {
          x = x+1;
      }
  }

  burn_cpu(99999999);

  </script>