Caitlyn crit damage bug [Gameplay]

So it's 6am Oce at the moment and i couldn't sleep. so naturally i booted up league and wanted to try some crazy stuff, one of the things i wanted to test was Caits passive at level 1 with crit damage runes. The passive clearly states that her headshot damage is increased by critical strike chance and critical strike damage. Only much to my surprise adding crit damage runes does not increase the damage at all. it's a flat 50% at level one without crit runes and it's still 50% with them. Kinda disappointed about that. it doesn't say anywhere that the effect of her passive is only increased by items? so i assume this is a bug? pls let me know if it isn't and there's something very obvious that i'm missing. Pretty sure anyone can go test this if they have crit damage runes.