Instrument error is usually something to do with the actual instrument (like the arm on the dial or something). The primary difference between IAS and CAS as I understand it is air pressure discrepancies in the Pitot-tube due to different angle of attacks and airflow disturbances due to the airframe. The POH of the Katana DA20 I regularly fly has a chart mapping IAS/CAS, with two different lines corresponding to different flaps configurations.
I'm no aeronautical engineer and this is a layman's understanding, so feel free to correct me. As I understand it, IAS (without instrument error) is the measured air pressure, CAS is the theoretical air pressure which would've have been measured, would it be possible to measure it perfectly, and instrument error is due to machining imprecisions or otherwise errors in the construction of the instrument.
Apparently the question wasn't if IAS without instrument error is the same as CAS, but why not use CAS throughout the POH. The easy answer is, since the CAS cannot be measured, but IAS can (to the extent that construction precision allow) and is displayed to the pilot, why should he memorize numbers which he'll never see? Charting IAS makes much more sense, and although they'll be offset by instrument errors, those are (generally) individual to the aircraft, and so it's the closest you're going to get in a printed manual which is not a one-of copy for a single aircraft.