His spell applies his passive, which is why its so effortless. By taking it away versus minions last hits require more skill from the Zed player to get each minion with out dropping last hits. He cant just spam his abilities with out paying attention while still getting 100% of the wave.
Secondly, the fact he doesn't have a ranged autoattack means nothing. You go to combo him, he >W>Q>E trades even, only your wave is gone because his passive so you are losing going even do to his superior minion aggro. Secondly, You will run out of mana before he runs out of health if he has even half a brain, and now you are losing trades to his combo while you go for "free" autos.
Also, by the time a Zed finishes a Hydra laning phase is generally going to be over. This is specifically a nerf to laning phase in is not meant to nerf his late game combat effectiveness or wave clear. Its specifically to nerf his 1 through 6 lane phase to open up counterplay options, by either pressuring him under tower where he would now have difficulty last hitting, and stop him from instant clearing the creep wave with out items (or at least slow down the rate he does it giving the opponent some time to even the wave out).
To be clear I don't have a problem with Zeds Power level, I have problems with how effortless it is to attain maximum potential on him. Why play any other champ when you can use Zed and just mindlessly spam and end up out farming and out pushing your opponent. He just has too much safety combined with effortless wave clear with practically zero counter-play.