Concern about Fiora dealing true damage based on % of maximum health
We all know how much people dislike how Vayne deals % max health true damage, and now Fiora is also getting that kind of damage, such that building defense against her will be almost pointless. Sure, Fiora isn't supposed to have the high attack speed that Vayne does, but (unless I'm mistaken) she can continually switch targets to deal that %max damage to multiple targets throughout a fight, whereas Vayne only does one target at a time.
Why was it decided Fiora would deal % max health in true damage, instead of flat amounts? Why make her passive uncounterable by health stacking?