Foreigners will often make that turn because a solid green light in many countries allows for a left turn when there is no on coming traffic.
I would need to find the traffic law in the DR on this point as it concerns the light in question by Playero to verify that a solid green light does not allow a left turn..
Ley 241-67 Ley sobre Tránsito de Vehículos
Article96.- Traffic lights.
a) Green Light or “Cross”.
1. The driver of any vehicle in front of this signal may continue in the same direction or turn, to the right, or to your left to enter another public road, as long as there is no sign or notice prohibiting such turns and that its movement does not close or obstruct traffic within the intersection.
What the AMET officer in question says who knows, and of course it is your fault if you collide.
Btw. because a gringo drives the car, it is assumrf his fault unless he can curse fluent in Spanish. Always