I think the ‘withdraw’ option if you look at it in a time frame perspective goes like this:
When subs withdraw, they do it while the battle continues. So when they don’t get to withdraw at the end of the battle, it’s not like they ‘sat around’ afterwards for a while doing nothing… they were actually fighting in the battle the whole time instead. But… if the battle continues, they use that time to slip away. If they slipped away AFTER the battle was over, it would illegally extend their movement, because they would be using a round of game time that doesn’t exist.
Make sense?