Superdrive for free with a new mac?

Hi,
i've read on this forum that a bunch of users get a superdrive for free with their new iMac/rMBP. How is it possible? Is there a promo code or is it like a "negotiation"?
I used the search option and google but i couldn't find an answer.
Thanks

it's not a matter of "greed", i need a dvd reader and i read here on the forum a couple of times that with an order of a brand new mac, apple will ship to you a superdrive for free. Then i was asking. If this is not true, then i will buy one, but if apple give it to me, why not?

p.s. this is an example, but i read it in more threads.