It didn't make much sense to me either. I started a thread about it in the model X charging section; the feedback I got was that people think the original Leaf EVSE isn't 100% standard compliant, and that's why the car won't recognize it.
(Back in 2011 a modified Leaf EVSE was pretty much the only way to get a portable 240V EVSE. I never had any problems with it and my Volt.)