Curious on Ahri's passive

We all saw Ahri's mini-rework after her dominance in S3 Worlds which in the eyes of many were unwarranted, but she's still doing fine and all. But one thing I'm really curious about is the change to her passive. Before, the passive would give her 35% spellvamp after reaching 9 charges (charges are gained by hitting any enemy unit with a spell, max 3 charges gained per spell). Now, it gives her (2+Champion level) + (9% AP) healing per enemy hit. Although her lane sustain remains largely unchanged hitting all the minions in a minion wave, it does nerf her combat sustain when she uses other spells to proc the passive. Before it was logical to use W when facing only 1-3 enemies because it was a single target spell and thus procced spellvamp to its full effect rather than the 33% reduction for AoE spells, now there isn't much benefit to using W for spellvamp unless there is quite literally only a single enemy standing on top of you. I don't get why Riot made this change, her passive wasn't really what made her somewhat broken back then, and I don't think changing it back would bring too much power to her, just makes her combat sustain and spell use more fluid rather than always trying to get a Q on the entire enemy team for sustain which is nigh-impossible when an enemy makes the tiniest effort to spread out.