Grid tie

The solar array is customer owned equipment so it needs to be on the load side of the disconnect. It also gets protection from the main breaker in case of a fault. The line side can be thought of as pretty much unprotected.
If the power is out, it is out on the line side too so the inverter will shut down either way. They actually "clock" from the line voltage. No line voltage, no clock, no output.
There are some companies that are coming out with an inverter that will work in an outage but I haven't seen one yet myself..

So the inverter freq. is triggered by the mains freq. so it's always in sync ? ( I had overlooked that issue )

Probably the main breaker is automatically opened if there is an outage on the mains. The utility doesn't want feedback from the solar system as a safety precaution. Dangerous voltages could be present on the HV side of the transformer- on supposedly dead lines.