Shingle roof systems are designed to channel water from the high points of the roof to the lower sections where it should exit the roof either into a gutter system or simply onto the ground. Shingle roof systems are not designed to hold water or prevent water from entering that runs or is blown uphill.
Ice dam related leaks are caused simply because the water could not escape from the shingled roof area. This is not a design flaw with the roofing system or an indication of a poorly installed roof. Ice dams are winter storm conditions which prevent the shingle roof system from shedding water as designed.