What decides weither a mana champion should have a skill that costs no mana?

I hate to jump the gun on this Ekko is op and overloading stuff, but the one thing that really stands out to me is that his ulti costs 0 mana.

I sorta gets Leblancs is a mimic so maybe.....? Olafs E definitely makes sense since he also damages himself though it didn't used to be that way I don't even know why sorakas W costs mana

But Ekkos really, is rewinding time that cheap? Balance wise it makes no sense to me and Thematic wise it makes no sense to me either.