Fiora's Passive doesn't work if enemy hasn't vision you, and that's silly.

See title. Why can Fiora only identify the vitals when the enemy has vision on her? Isn't it also a bit unfair Fiora gets to know you've warded the bush she's hiding in because vitals appear on her enemies?

I get the argument that otherwise, enemies'd be warned Fiora's near, but can't you just make it so you cannot see Fiora has vitals on you if you cannot see Fiora herself? Lets her use the brush for attack without sacrificing losing her passive.

Or is there a good reason she can't use her passive unless enemy has vision on her?