What roof and gutter deicing cable is designed to do.
Roof and gutter deicing cable creates a drainage path for meltwater to leave the roof through the gutters and downspouts. It is not a system for clearing all snow and ice from the roof or removing a roof snow load.
The Ice Shield instructions specifically prohibit using the cable to clear the roof of snow and ice. Install it before ice dams form, following the instructions for the exact cable and roof type.
Source: Ice Shield roof and gutter deicing installation manual, Safety Information and Purpose of the Heating Cable, pages 2–3.