RDF Schema - question on implied typing with rdfs:Class

Hi,
 
I am new in the RDF scene, and I have read most of the related W3C
Recommendations and the Primer.
 
I have a simple question about RDF Schema, with probably a simple answer:
In the Primer I read that ALL classes need to be typed with rfds:Class. So
for example:
    ex:MotorVehicle          rdf:type                rdfs:Class .
    ex:PassengerVehicle   rdf:type                rdfs:Class .
    ex:PassengerVehicle   rdfs:subClassOf   ex:MotorVehicle .
 
Is there a reason why the subclass ex:PassengerVehicle doesn't inherit being
an rdfs:Class from the superclass ex:MotorVehicle?
 
Regards,
Hans
 
____________________________
Hans Teijgeler
co-author of ISO 15926-2 <http://www.infowebml.ws/ECM4.5/ECM4.5.html> 
author of ISO 15926-7
website www.InfowebML.ws <http://www.infowebml.ws/> 
e-mail hans.teijgeler@quicknet.nl
phone +31-72-509 2005