Fizz having the 20% damage amp while Ahri gets it removed actually makes sense.
Ahri was not intended to be an assassin/burst mage but she was made to be one since they don't want you being a cancer tumor to this game and using DFG plus WRRR to get a kill. Her kit can easily be changed to be a kiting mage with decent pick potential.
Fizz was intended to be an assassin. His entire kit was designed with the idea that he's gonna one-shot somebody then either die or attempt to run away. The only thing his kit could be changed to is a bruiser, but I'm sure most of us don't want a tanky fish who still does high sustained damage and is impossible to catch rampaging across the rift. Fizz's damage was reduced in general, and this makes him a fair bit healthier since he has to land the shark to get a kill. This means you won't get bullshitted by a Fizz missing his fish and killing you anyways nearly as often as you did before.
tl;dr Ahri's intended role doesn't line up with a 20% damage amp while Fizz's does