About the Approach 1, Representing Classes As Property Values on the Semantic Web

Dear all, 

I've a question about the example for the Approach 1 in Representing Classes As Property Values on the Semantic Web. Below is the text extracted from the document:

{{

For instance, we may want to define a class of all books about animals—BookAboutAnimals—that 
our animal books will be instances of and we want to restrict the range of 
values for the dc:subject property for the BookAboutAnimals 
class to the class Animal and its subclasses. One way to define 
such restriction is as follows: 

:BookAboutAnimals
      a owl:Class ;
      rdfs:subClassOf :Book ;
      rdfs:subClassOf
              [ a owl:Class ;
                owl:unionOf ([ a owl:Restriction ;
                               owl:onProperty dc:subject ;
                               owl:hasValue Animal
                             ] 
                             [ a owl:Restriction ;
                               owl:onProperty dc:subject ;
                               owl:someValuesFrom
                                    [ a owl:Restriction ;
                                      owl:onProperty rdfs:subClassOf;
                                      owl:hasValue Animal 
                                    ]
                          ])
              ] .

}}

My question is,  why do we have to bother defining a union class above? Since a class is a subclass of itself, so from my understanding,

                             [ a owl:Restriction ;
                               owl:onProperty dc:subject ;
                               owl:hasValue Animal
                             ] 

has been subsumed by 

                             [ a owl:Restriction ;
                               owl:onProperty dc:subject ;
                               owl:someValuesFrom
                                    [ a owl:Restriction ;
                                      owl:onProperty rdfs:subClassOf;
                                      owl:hasValue Animal 
                                    ]
                             ]


Is there anything I missed here? Thank you!

Best,
Mark