The answer is extremely simple. The inverter requires a constant/steady power source to operate. It cannot be powered directly from the panels. In hybrid and fully isolated systems, the panels charge the battery, the battery powers the inverter, the inverter runs your home.

In a grid-tied system there is no battery. When you lose the grid, you lose your stable power source to power the inverter which is why it shuts down. Without having a hybrid system that has the battery bank, the panels CANNOT power the load (your home) directly. It is a technical issue, not a legal one.

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Current stable:
1995 E320 149K (Nancy)
1983 500SL 120K (SLoL)

Black Sheep:
1985 524TD 167K (TotalDumpster™)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)