The title says the point, really. But I'll expand.
I use Lux support. I buy spellthief's Edge for the extra AP. I've noticed that if I hit my E only both the support and the marksmen, I'll only proc my spellthief's passive once.
If I use Nami and use my ebb and flow to hit both of them, I'll proc the passive twice.
This leads me to believe that the item is set to give me gold when I do damage, and not for each time I've hit a champ.
I want to be rewarded for hitting both of them. I also don't want to be encouraged to just poke ONE opponent with my AoE spell just to get the gold as I race my opponents who take the faster gold items.
I'm not sure if this is selfish or not and I don't know if it would be broken if changed. I just feel like it's such a waste for me to not be rewarded for a good play on an item that encourages poke.