It's because of the very low margins in these products - the retailer is having to pay the finance company a $ value for the interest free term and with many low margin products there is no profit in it for them.
As Steve has pointed out, most retailers exclude Apple products and gaming consoles due to the very low margins on them.
Apple provide the low margins on their products, and competition is the reason for the low margins on the consoles. Also, there is no such thing as interest free, someone is always paying for it. In the case that we do not have to, then the retailer pays it.
I've found from experience if you have a real good credit rating with the likes of GE Money from years of business with the likes of Noel Leeming, sometimes they will try to get a interest free deal for you and sometimes get them :) But yeah the thing with Margins is correct, a previous job of mine gave me the ability to get things at Noel Leeming at cost price, when I checked out a gen 2 4gb ipod a year ago, the discount only worked out to be around $15, so yeah there is def nothing in it.