FileVault is a special kind of thing, Ed, and can't be backed up this way. It's always going to back up the whole thing, because FileVault is stored in a sparse image. We operate on it in exactly that way.
Note, too -- as we say in the User's Guide, you shouldn't be backing up a FileVault protected Home folder when logged into the account.
My suggestion, really? Don't use FileVault unless you really have to. Instead, use a product like
Knox to encrypt only what you need to.
Hope that helps!