How is champions with a strong early game buying early game items an exploit? It sounds like good strategy to me. They do cost you lategame, so stacking them only pays off if you can take advantage of the early power (i.e. the stats you get allow you to snowball more gold than you'll lose from having to sell them later).
It's simply a matter of altering your build to handle the problem. If you are getting pushed around in lane by an AP champ, it's perfectly legitimate to grab an early negatron cloak or some mantles to compensate (mantles if you can build them both into something). This game isn't supposed to be build the same items in the same order every game. That's boring.
Here's a good example. I played a ranked game yesterday where I had to mid against Ashe (with full armor pen runes) as Zilean. To top this off, they had a roaming AD Eve. I realized I wouldn't be able to stay in lane without dying unless I bought some early defense, so I grabbed a glacial shroud as my first item. This kept me in lane and kept me with the farm I needed to not let them get a large advantage from their early game aggression. I survived to mid-late game and we crushed them because I reacted properly.
If there's a champ that's particularly strong with Doran's items, it is more likely that the champ is just too strong in lane than that Doran's items are too good.