If you'd redeemed the codes Apple gave you as a registered developer, you wouldn't have been able to buy Lion – it was already purchased by your last redemption

If you'd redeemed the codes Apple gave you as a registered developer, you wouldn't have been able to buy Lion – it was already purchased by your last redemption code and, like all previously purchased App Store apps, clicking that button would simply have re-downloaded the app (or, in this case, Lion's installer).