I am not sure if I understood Your question correctly - do You want to know why piston engines are prone to shock cooling or why turbines are not?
Anyway, let me offer the following thought: A turbine engine has no heat sink surfaces exposed to free flowing air. Cooling takes place only where the operating temperatures are beyond what the material can handle - in the combustion chamber and the turbine section. (Depending on the engine type, the oil system may or may not have an air-cooled oil cooler as well) The cooling air in a turbine engine is taken from the high pressure compressor stages and has a temperature of a few 100°C - this is sufficient for cooling in areas like the combustion chamber or forward compressor stages that would otherwise face temperatures beyond 2.000°C.
Now in an idle descent, the engine is still running, combustion still takes place and the compressor still delivers hot high pressure air for the cooling systems; both mass flows are lower than in other phases of flight and the temperatures are a bit cooler of course. On the Dash 8-400, normal ITTs are around 350°C in descent compared to about 650-750°C in climb.
In a piston-powered aircraft by contrast, the heat generated in an idle descent is strongly reduced, but the airflow across the constant area of the cooling fins is not (assuming no cowl flaps are installed) due to the typically high speeds in descent.
So the shock cooling is caused by the strong airflow that removes more heat than the piston engine provides in this situation. On a turbine by contrast, the engine produces less heat as well, but as the cooling air flow from the compressor is still much warmer than the outside air and provided at a lower mass flow rate, the ratio between produced and removed heat is still rather well balanced.