Maybe it treats it as though the player that initially created the courier is the one "announcing" items on the courier.
Whenever an item is alt-clicked on the courier it is attributed to the courier owner, this applies to courier abilities as well.
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Any game:
Buy wards or any consumable and alt+click the item on courier, see if it says "observer ward ready" and has another guy stating that they are ready. Potions/tangos dust smoke it all seems to show someone else saying that "its ready" when alt clicking the item you bought while on the courier.