PVC may be non-conductive, but its edges are still sharp enough to damage insulation.
I think that's the reason behind 300.5H.
BTW, I always call for metallic elbows in underground PVC runs because the rope that pulls in the conductors can grind an edge in a PVC elbow during a tough pull. I've had cables fail to each other on an U/G feeder for just that reason before I adopted that little trick.
