Why UserDatabaseRealm may use only resource from global JNDI but DataSourceRealm may use both global and local JNDI?

[outsider404 Gazeta.pl]

[Tomcat 7.0.x]
I wonder why UserDatabaseRealm use always global JNDI:

==========
Context context = getServer().
getGlobalNamingContext();
database = (UserDatabase) context.lookup(resourceName);
=========

but DataSourceRealm is more flexible:

==========
if (localDataSource) {
  context = ContextBindings.getClassLoader();
  context = (Context) context.lookup("comp/env");
} else {
  context = getServer().getGlobalNamingContext();
}
===========

Is it intentionally, for example for some security reason or it is open
improvement option for UserDatabaseRealm?