Why does Riot think manaless AD champs are OK?
For Yasou and Riven that's pretty easy:
Both champions essentially auto-attack with their abilities.
Auto-attacks are manaless.
A very large number of AA steroids and on-hit effect skills present on champions that use mana also happen to be manaless
(
and more)
Yasou and Riven have kits centered around the idea they auto-attack things to death with what are essentially creative auto-attack steroids, and because that's the core of their kit and what they're balanced around it makes sense that doing so would cost exactly the same amount of mana as it does for everyone else (0).
Also, Yasou and Riven are essentially marksman with melee range. They need something to make up for the loss of a 550 range basic attack, which is a pretty huge hit to take for their respective kits. They need a lot of power to make up for that loss, which is why the kits of both look overloaded.
As for Zed:
He's a ninja, all the Ninja's use energy.