[JohnM]

That's actually intentional, though undocumented, behaviour. Imported impulse responses will have been generated by some other measurement system so it would usually not be appropriate to apply the REW soundcard cal or mic cal to the data being imported, as REW cannot know what calibrations or corrections had been applied when the impulse response was captured. I'll add a feature request to have the option of applying either or both cal files to imported responses.