HIIT is supposed to kick your butt enough that you don't want to do anything afterwards (except cough up a lung...)
It is my understanding that if you do cardio (HIIT or steady state), it should be done after weight training or on your off days. If you do cardio first, it depletes all the glycogen in your muscles before you even get to lift any weights. Your muscles are then too tired to put forth the effort required for an effective weight lifting session. And as rixatrix mentioned in another thread, HIIT after weights is also helpful in flushing out lactic acid from your muscles.