When you buy Lion (other than with a new Mac), what you get is a licence to upgrade from Snow Leopard. So if your Mac is still running Leopard, you face a double whammy.
A further problem is that Apple's MobileMe service finally closes at the end of June, and the company is keen to migrate as many users as possible to its successor, iCloud. But iCloud requires Lion, and upgrading to Lion requires Snow Leopard.
So when MobileMe users log in to their accounts they are now presented with the offer of a free copy of Snow Leopard as a stepping stone to Lion.
This doesn't help owners of Macs that don't meet the Snow Leopard requirements (eg, PowerPC-based models). And its only a marginal courtesy for those who can run Snow Leopard but not Lion.