System disallows granting System.GRANT

[Josh Elser]

public boolean canGrantSystem(TCredentials c, String user, 
SystemPermission sysPerm) throws ThriftSecurityException {
  authenticate(c);
  // can't grant GRANT
  if (sysPerm.equals(SystemPermission.GRANT))
    throw new ThriftSecurityException(c.getPrincipal(), 
SecurityErrorCode.GRANT_INVALID);
  return hasSystemPermission(c, SystemPermission.GRANT, false);
}
Why don't we allow System.GRANT to be given to other users?