Comment 6 for bug 975867

After carefully inspecting this issue this seems to be the expected behavior.

In normal runlevels "halt" just calls "shutdown -H now", which is supposed to simply halt the system without poweroff.
Alternatively the "poweroff" variant of the command calls "shutdown -P now", which actually powers off the machine.
(and halt -p is equivalent to poweroff)

From "man shutdown":
       -H Requests that the system be halted after it has been brought down.
       -P Requests that the system be powered off after it has been brought down.

From the reboot/halt/poweroff source:
  switch (mode) {
  case REBOOT:
   args[i++] = "-r";
   break;
  case HALT:
   args[i++] = "-h";
   args[i++] = "-H";
   break;
  case POWEROFF:
   args[i++] = "-h";
   args[i++] = "-P";
   break;
  }

I guess we just got used to systems that powered down when calling "halt" when they really shouldn't, but I have to say it's been a while since I encountered a system behaving differently...

PS: found some related links:
more info here: http://ubuntuforums.org/showthread.php?t=1968626
duplicate bug: https://bugs.launchpad.net/ubuntu/+source/upstart/+bug/991997