It is a sound tactic if:
A) The retreating unit has a ranged attack, like the air shrill's shock, or the brigand's throwing knives.
If the retreating unit has significant regeneration like the troll warriors.
For the rest of the cases, one could rationalize it as "morale failure", but unless player units got that, it juts gives us an advantage that we don't need. And I, personally, do not think that this particular game needs morale.