What is the real story behind "free add-on" items?

What is the real story behind "free add-on" items?

Ali Nasri (Computer Specialist, Mac 911 Inc.) gives expert video advice on: Is there a good time to buy a computer?; How do I know if my computer salesperson is doing a good job?; Is it a good idea to buy my computer online? and more...

Add-ons almost always have a mail-in rebate associated with them. So you go and buy something. "Buy this computer and get a free printer" and then in fine print, "after mail-in rebate" which you almost never see. You'll forget to mail it in. I am not much for those free add-ons. I would rather get the product that I am getting for a good price than have to worry about dealing with those mail-in rebates. The computer stores are counting on that you don't mail in those rebates.