Why does File Vault II need a recovery partition to be able to be used?

I was looking at the partition and recovery information for Lion and I read that "A recovery partition must be present to be able to use File Vault II Encryption".

Any idea of why that is? It would seem you would have an open security breach in being able to start the machine on the recovery partition and then use that access to access the encrypted partition. Unless there si something I don't understand here.

Jim