Gentlemen,
If you look at the club home page under the club rules for gaming you will find
5.1.4 Withdrawal of all (or a substantial portion of) forces from the battlefield, unless specified in the scenario as a victory condition, will cost the withdrawing player a 2-step reduction in the level of victory. Removal from the map of individual units, routed behind enemy lines by the game engine, is allowed. (This rule is waived in the case of campaign scenarios.)
These rules were in place when I joined the club in 2000 for the reason stated by Gen Mihalik, several of the BG scenarios gave the rebels an early advantage, if they were allowed to attack until their army was worn out then retreat off the map they could often build up a large enough casualty edge to prevent the union player from winning after his reinforcements came on.
The rule was amended when the HPS campaigns came out since it would often make sense to leave the field in a scenario to save your army for use later in the campaign.
As to individual units, the rule is clear that units that are cut off from your main army can leave the field. I once suffered a minor defeat in a BG Antietam scenario when my opponent retreated an isolated unit off the field. He offered to call the game a draw but I saw no problem with his action and we registered it as a minor victory for him.
Gen. Ken Miller
AoP
