Why are CT's(Current Transformers) on an ammeter / energy meter connected to ground?
100% sure answer may need much digging but

It is almost certainly for safety reasons.

A current transformer output produces its result in splendoured isolation - it floats relative to the universe (or ground or line or ...). Stray leakage and capacitive coupling MAY cause it to float to undesirable voltages so referencing it to something is a good idea [tm]. 

Connection to phase would achieve this - it will still function the same BUT this may make your eyes water or worse if the insulation is not perfect or you find some exposed part of the circuit to touch. 

SO connecting it to ground at some point in its circuit may make your days longer on the face of the land - or those of your client or family or ...