Agreed - there is a fundamental difference between a unit marching off to attack the enemy, not itself being under attack, and a unit that is under fire. Presumably LG applies a morale test in both cases but I assume that in the first case, it would be unusual for a unit to fail unless it had already taken casualties or was facing bad odds. Whereas once a unit is actually being attacked, then its morale might change more quickly and you might wish to keep a careful eye on it (assuming there is time for that sort of micro-management).