I don't know what the codes say in Maine, but no where else in the country - or the world - do I think that is standard practice. Neutral is not ground, rather it is somewhere half way between L1 and L2 on a single phase AC circuit. Should be near ground potential, but since it is carrying lots of current it could be many volts away. Green ground should carry no current except in the case of a fault.

If you have green and white connected at the receptacle, you don't have a grounded system; if green is further connected to the metal boxes as required by code, you have a dangerous system.