There could be other reasons why they retain data. For example, it could be to maintain data integrity. Let's say you've sent messages to people, left comments, written on walls, etc. When you delete your account it's going to be much easier to flip a flag somewhere to indicate that the account is no longer active than it will be to remove any trace of your existence there.
Think about it. It's not an especially easy problem to solve. The data that people put on the site can relate to each other. If you remove some pieces those relationships break and context is lost. Not to mention, finding and removing the data could be an intensive task, especially when it could be scattered across many shards.
There's also the matter of how remaining users will feel when data you've left for them suddenly disappears. If you've sent me a message and I chose to leave it in my inbox indefinitely why should it vanish because you've decided to delete your account? You sent it to me. Isn't it mine now?
The reasons for abandoning an account are independent of the reasons for retaining the data associated with that account so they would be just as applicable in the case of death. That said, I have no idea how Facebook would handle the situation. It really seems to be an issue of control of the account and not so much data retention.
