You paid for your current OS when you bought your computer. As far as I know you didn't pay an extra 20$.
Apple publically announced that any computer purchased July 11th or later would get the copy for free, so while you didn't pay the $20 directly, you are entitled to that, otherwise Apple would be guilty of false advertising. It was a promotion, and businesses do that.
Not getting it immediately, however, isn't a guarantee. It even says on the page that it takes 24 hours to send the code once the form has been validated. The sucky thing here is that none of this information was published before this morning when the purchases were made; before today it simply said "Come back to this page when Mountain Lion is released in July and get your free copy of Mountain Lion".