That is related to the way NFS maps and authenticate users.
VMware does not know how to use something else than UID 9 (root) to mount and access a datastore, and usually that wouldn't make any sense to do otherwise anyway since datastore are dedicated most of the time.
So, VMware mounts the volume with UID 0, and there is a special security parameter in most NFS servers that determines if UID 0 is to be mapped to another user or if it is allowed to do writes. In that case, you just need to say that, yes, I want root to be able to use the export from the given IP addresses (which are all your VMkernel NFS IPs)
Please correct me if I am wrong. As per your reply what I understood is that UID 0 is only used for mapping but not for read/write? and this is NetApp's behavior not VMware's as NFS server is NetApp's facility?