If Fiora Riposte is up, you'd have to be legitimately fucking brain dead to use any more than one thing on her. There's probably at least half a dozen things a team has she is pretty much required to parry to survive. Use the least important one out of those, force out Riposte, then hit her afterwards.
Okay so then stand behind the tank or have the tank do it because it doesn't matter the slightest fucking bit if they get stunned briefly.
Your comparison is irrelevant. For Fiora to land the slow without actually parrying, her enemy has to literally just stand inside of the clearly outlined hitbox on the ground for 0.75 seconds until she fires said slow (the thrust animation of her sword). You don't have an excuse to stay there for no reason if you weren't doing anything.
If you were doing something, that means she successfully parried so there is no problem.
Kayle's ultimate is instantaneous cast on whoever it is, including herself. There is literally no additional effort or thought involved. You simply either ult yourself when low in a 1v1, or ult the carry in a teamfight, whether it's yourself or someone else. It's instant benefit and instant reward.
A better analogy would be complaining about Lux E still exploding automatically even if she dies after like 4-5 seconds. "WTF WHY DOES IT STILL DO DAMAGE EVEN IF SHE'S DEAD"
Okay so then why are you standing inside the E for no reason if she's dead? You can literally just walk out before it self explodes.
Same thing. If you're not doing anything, you have no excuse to stay inside the W hitbox. If you were doing something, she parried and thus gets the slow off.