Upload & Sell: Off
"What was wrong with the instant rebate system upon purchase?"
Instant rebate means 100% redemption.
Mail-in in rebate means much lower redemption rates, perhaps even < 50%, as people forget or fail to complete properly. So the overall cost of the 'campaign' is much lower for Canon.
Add to that the stupid cash cards they give that are difficult to use. What happened to the good ol' days when you received a check in the mail?