I have no Idea why it bugs me so much what do you guys think?
I'm pretty sure having damage split between AP and AD scaling bugs everyone.
As for why Riot did that though, having damage output split 50/50 between physical and magical is the best possible set-up for any champion that's going to predominantly concentrate on building defense.
That glass-cannon Zigg's or Draven can't just purchase a Negatron Cloak and become a mega-tank against your damage. They can't purchase a Chain Vest to counter you either.
However, you can easily itemize against those guys by grabbing the appropriate counter-item.
Inversely, they're going to get a lot more out of a glass-cannon adventures path than Alistar.
So, it pushes tanky champions to build tanky and damage champions to build damage.
Considering items play such a massive roll in champion rolls this strikes me as good design.
Alistar isn't alone in the split damage side of things either - pretty much every tanky DPS is a pretty solid 60%/40% split. Nasus, Udyr, Shyvana, Dr.Mundo, Darius, Warwick - all of these champions aren't easy to counter-build by any stretch.