You have already received our maximum number of allowed shipments. Please download the CD image and record it yourself.
I don't know there is a limitation. As a overused Ubuntu user, I was banned to view my request history so I don't know what the limitation is. I requested only ONE CD at a time, but not every release, and I hit the limitation after I've been using Ubuntu for 5 years.
From Koala, I won't be a burden of Canonical. I will download image file and burn CD of every new release of Ubuntu. I will continue to tell every possible Ubuntu user the free request service, because I know, before they hit the limitation, they are very likely already a Ubuntu lover.
Enterprise users are new target of Canonical, personal users shouldn't expect too much from it anymore.
UPDATE: Better late than never, here is the official explanation from Canonical, Keeping Ubuntu CDs available