Couple of reasons.
1. As stated in the post if you don't click to include postage when you order you forfeit the rebate.
A lot of people don't bother to read the page which states you need to do this.
2. I lose money with a $10 rebate so I'm not going to change the page when I want to recoup some of my production costs. It's too much of a hassle to change it and then change it back after. So it's easier for me this way.