The root account in Ubuntu is not disabled. It is not meant to be logged in directly with a password. It is configured in such a way that you need to sudo from a non-root account. In this way, there is some information as to who started each root session.
You can trace why su --login does not add the sbin directories to the $PATH of the non-root user. My recollection is that it follows the Unix tradition of not adding system directories to the $PATH of non-root users.