Perhaps becasue the entire point of the ATZ is to provide greater safety to circuit traffic? I agree that occasionally at that point in the circuit the aircraft will be just outside a 2-nm ATZ, however as a general course it should be just within the ATZ on a standard circuit pattern. By chance I was chatting about this with someone from SRG recently, he having introduced the subject ciritcising circuits being flown large enough to be outside the ATZ.