I could see it as, not that he's being taken a peg down or two because he is compared to Lucifer, but rather because Heaven frowns upon making deals with the devil, and maybe God returns or something like that and while he understands that Cas is trying to protect everyone, he still has to be punished according to Heaven's rules, but he loves his son, the son who stood up for humanity and done so much, so instead of sending Cas into the pit like what happened with Lucifer, Cas is sent to Earth as his punishment.
But Cas has the Winchesters there on Earth to help him adjust, so it's not really a punishment as much as letting Cas taste freedom and live life. I can see being a human as both punishment and a prize. It's punishment that he's not an angel anymore or have powers, but on the other hand, it's a prize because he's living and sharing his life with people who will love and care about him, and people whom he loves and cares about very much.
