User blog comment:Astanas7/Let's talk Sion without much formatting/@comment-10764129-20140122072640/@comment-5993740-20140122074514

This isn't just ADC's but a lot of AD champs, like Wu's scaling on his Decoy. It's so that items that give a bit of AP can be used, like Triforce. This seemed more obvious when WoTA was an aura, but since it's not anymore, it really is rare. Most ad champs without an autoattack modifer won't touch Tri/Gauntlet, but the scaling in still there.

Maybe it's so when people troll build AP, they'll do slightly more damage.