Common ground on a switch mode power supply
If you don't care for EMC, you can solder the green/yellow earth wire to both the holes. Because an electrical connection may not serve as mechanical connection, you must secure them mechanically, so they don't come loose.   

Why they didn't connect M1 and M2 on the PCB?
Likely for space: a chassis trace on the primary side of the PCB (having live parts) requires clearance, double isolation, which is relative much space.
And maybe for meeting the EMC requirements