You ice the intake because it's a big chunk of aluminum that the intake air goes through. If you where to "ice" the whole engine, you would warp lots-o-stuff.
Ever heard a valve pop? Get a 90 Mustang GT with open headers, drive around at WOT for about 30 minutes, then turn off the car when it's 25 degrees F outside. That cold air rushes in and hits the exhaust valve. It makes a loud pop.
