We do ship only one implementation of those classes so in fact we wouldn't need to set the default values for the FQCNs and the security configuration in domain.xml and standalone.xml for 99% of the users would be simply
<subsystem xmlns="urn:jboss:domain:security:1.0"></subsystem>
I was just trying to show the complete model with all the attributes filled with their default values so we can evaluate the options available to customize security.
Some customers do need to use custom implementations so there is not much way around setting the FQCN of the implementation somewhere in the configuration.