I mean, it makes sense in a roundabout kind of way not to destroy DS' stored in a position when the position is deleted. For example, let's say you have an inventory list, with each position being a map storing some data about the item. You then want to, I dunno, show all the armour in your inventory. You create a temporary list, loop through the inventory and store the list references for all the items that are armour in your temporary list. The player can select stuff from this armour list, so when they click on the armour item, you delete the position from the list. If that destroyed the armour ds_map, you'd lose the armour from the inventory (which, in essence, would force you to clone your inventory list AND create a temporary list to get this kind of thing going, or clone each map, or some other vaguely more roundabout way of doing it). But because it only deletes the list position and doesn't destroy the map, your inventory list is safe.
However, very much yes, the documentation is sorely lacking in some of this more obscure functionality and the number of hours I've wasted trying to hunt down a bug only to later be told by someone in the know "It functions in x way but that's undocumented" is pretty obscene.