Inter-penetration by Light Troops

As I understand it, when ordering Light troops using Move-Fire-Move and Order Light Troops, units are allowed to pass through freindly units. If, however, Light Troops are ordered as a unit by section cards they loose this ability.

I think I understand the game mechanism for this, but it seems strange that they don't keep the ability.

Yes you are correct, although light mounted units don't get the pass through ability.

Don't know why it is not a general abilty for other cards?

If it were I'd be able to better manage my light infantry rather than have their evade move blocked by friendly Medium and Heavy Infantry eager for the fight!