Digital ICE uses IR to detect IR scratches and dust that block IR light.
The emulsion on C41 colour films are IR lucent, so dust is can be identified as 'shadows' on the IR image. This allows digital processing for removal of the dust, etc.
The silver crystals of the emulsion is not IR lucent so ICE will think the entire black and white image is 'dust'. So using ICE on traditional B&W softens the image.