Okay, this has been discussed and explained a number of times before. Even Helen at Adorama has provided input in the past. Basically, Canon has the right to set a Minimum Advertised Price and they require retailers to honor that or they can't participate in the rebates.
The key is the "advertised" price, which isn't always the actual sales price. I don't pretend to fully understand all the rules, but it's what enables retailers to offer special deals through sites like this and through eBay without violating the minimum advertised price rules. It's also what forces retailers to use the "add to cart" option to get to the real price.
Bottom line: the market sets the price. Canon tries to control the prices through Minimum Advertised Price and rebate programs. They are doing this to protect their dealer network, but the dealers can only sell the product at the price the market will bear.
Rebates or not, the same rules always apply: be a smart buyer and comparison shop using the online resources available to you. Know what's a fair price and balance any savings against the reputation of, and your own experience with, the seller.