I think the purpose of the selection when you cast the spell is to know wich army will move. But obviously the Idols in my cities won't move far away

Maybe the purpose was to allow the teleport of armies standing in cities to an other location, but then I have only one Idol suggested and not as much as I have cities. Anyway, if I cast it using my Idol the army in the city do not move to the location is select, at least the timeout is activated for the Idol and on the second cast I have no selection screen and my sovereign army is the one moved (my sovereign being the only hero with this spell).

Also, as shown below, the caster selection windows may need some better placement, the partial overlaying on the minimap is not very clean.