Why do you get upset if I start Cull on a mage?
First of all it doesn't build into any AD item, so you have no reason to throw a fit over that.
Secondly, runes give a lot (and I mean, A LOT) of adaptive stats, and at level 1 even mages get more value off of AD than AP (better autoattack damage for trades/poking and easier last hitting). (Btw on the first back I'll likely buy some AP, which will morph that bonus AD from the adaptive stats into bonus AP) (As a mage I'm not an assassin, I'm not banking on a level 3 all-in, thus I don't need bonus AP that early and that badly)
Thirdly, I expect to hit 100 cs well before having to sell my starting item, and Cull happens to give a nice amount of free gold when that happens.