I initially wondered about this myself; this is my first car featuring forced induction.
As the other members have stated, it isn't required that you let your ST idle for a short period before shutting off. I generally let mine idle long enough for the radiator fan assembly to finish its current cycle. During colder months, I'll let the engine idle for 15 to 30 seconds.
I've read through the manual about this as well. It states that we should allow the engine to return to idle speed before turning it off.