Why are champ's penalized against towers for going hybrid?

You deal either your total attack damage, or your base attack damage plus 40% of your ap, which ever is higher (and minus its armor either way, and unaffected by penetration). Most hybrid champs still build AS, so they aren't too bad at taking turrets, but they are strictly worse at it than if they'd gone AD or AS AP. This is part of the reason why ADC's are in every game without exception, as I whine about here. (RiotRepetoir commented in it)