If you look at spell vamp, it is 1/3 as useful on AOE spells. I understand that this is because AOE has the potential to do lots of damage. My question is why do empowered autos get the extra life steal as well?
AoE specifically does a lot of damage because minions and monsters take true damage from basically everything.
Single-target damage usually has to deal with 30+MR and armor on champions, or the dissadvantage of being out the resources (time and mana) when using it on a minion, opening yourself up to a lot of counter-pain.
So - AoE can allow one to do substantially more than x3 damage. AoE skills are usually weaker due to longer cooldowns over their overall damage as well, allowing AoE nukers like Annie to stay pretty close in damage to single-target bursters like LeBlanc.
If you compare the champs 
and 
they have the same passive relative to their kit.
Morganna is a considerably safer lanner than Nasus, she can be immune to CC, has a shield capable of blocking magic, and has near Ziggs level range concerning wave-clear.
The only time she has a hard time in lane is if she's up against an AD assassin with 0 reliance on magic damage, CC, and endowed with the mobility to negate her range in a 1v1 scinario.
Nasus is a risky pick into any champion with in-lane kill potential. The only time he has Morganna-level safety is if you run someone like Zac into him.
I'm not buying this advantage Nasus you're trying to give here.
Also - most modified auto-attacks are essentially planned critical strikes. You get life steal on an IE crit with a 150% AD ratio, so why should (arguably lower) auto-attack boosters not apply life steal?
Finally - life steal has a ton of awkward interactions and applications. At least empowered auto-attacks stick true to the fairly digestible "life steal heals you for a % of the auto-attack damage you inflict". I don't see ~eh reasons being strong enough to change sensible, easily understandable rules to convoluted and archaic ones. Such a change would be like Blinds not stopping on-hit effects on auto-attacks.
? It specifically states the extra shots apply on-hit effects.