Why? If you wanted to deal with the abuse to kill bosses with minions after dying, you should make them deal no damage, like POE does. Right now it’s a huge annoyance to resummon minion army every time you die. I play abomination build and resummoning it takes 30 seconds. I don’t want to push corruption because of that, because dying now makes me want to ragequit.
Making them all die is obviously easier to implement and less prone to abuse.
Given that, then they could make it so that summoning minions in a safe area would cost no mana.
It is a problem, though. Due to the way the system works, mana refunds aren’t an option. Which is why you don’t get mana back when you try to summon in a place where they can’t be summoned. I had that discussion once with Mike and it’s not that easy.