Well it makes sense - at the moment you can buy just about any Warmoth body/neck combination and they'll fit together. Everything is made to extremely tight tolerances and it's all simple and straightforward.
If they did an AANJ, even if they used the same heel shape on every body (which probably wouldn't work anyway), they'd have to offer all the neck shapes with an alternative heel design. Then for stock purposes they'd have to decide how many necks to make in heel shape 1, how many in heel shape 2....
And there's no "industry standard" AANJ shape, so would a Warmoth neck fit an Ibanez? Washburn? ESP? I suspect they don't want to make their lives too complicated.