This differs from a sale in that you pay the sale price and not the regular price, followed by getting a rebate. Sure it's mainly semantics but that's most likely what's happening in the case of an "instant rebate".
I wonder if there are tax implications on the seller's (either retailer's or further up the sales chain) side which make the "instant rebate" form more attractive?
Given that one of the appeals of rebates over cost reductions is that there's some float on the rebate cash, perhaps with instant rebates there's some "float" on the item that the manufacturer holds until sold? Because the other appeal with rebates - that not all eligible customers will actually bother to redeem them - won't be the case with the "instant" variety...