Make an required attribute optional, by restriction?

Hello all,
 
I do hope that this has not been covered before, but I have
got another derivation problem.
 
Given the following (simplified) schema:
################################
 
<xsd:schema xmlns:xsd=" http://www.w3.org/2001/XMLSchema
<http://www.w3.org/2001/XMLSchema> ">
   
   <xsd:element name="Liste">
      <xsd:complexType>
         <xsd:sequence>
            <xsd:element name="Firma" type="FirmaType" />
         </xsd:sequence>
      </xsd:complexType>
   </xsd:element>
   
   <xsd:complexType name="ListType">
      <xsd:sequence>
         <xsd:element name="Name" type="xsd:string" />
      </xsd:sequence>
      <xsd:attribute name="MId" type="xsd:integer" use="required" />
   </xsd:complexType>
   
   <xsd:complexType name="FirmaType">
      <xsd:complexContent>
         <xsd:restriction base="ListType">
            <xsd:sequence>
               <xsd:element name="Name" type="xsd:string" />
            </xsd:sequence>
         <xsd:attribute name="MId" type="xsd:integer" use="optional" />

         </xsd:restriction>
      </xsd:complexContent>
   </xsd:complexType>
 
</xsd:schema>
 
I was stunned seeing this schema being validated against an instance
without any mistake. 
 
My first question is: 
When deriving a type by restriction, I do not think
that it is a valid restriction for an attribute to an allow an
use="optional",
if the same attribute is required in the base type.
What am I missing here?
 
 
I must admit that it is very hard for me to read the Structures. It makes
it even harder, because I´m not a native speaker. But enough of that.
I thought that the following exerpt 
 
Schema Component Constraint: Derivation Valid (Restriction, Complex) 
2.1 If there is an attribute use in the {attribute uses} 
    of the {base type definition} (call this B) whose 
    {attribute declaration} has the same {name} and 
    {target namespace}, then all of the following must be true:
2.1.1 one of the following must be true:
2.1.1.1 B's {required} is false. 
2.1.1.2 R's {required} is true. 
 
would be an affirmation of my problem, I mentioned in the first question.
But regarding the facts I´m facing, this can´t be true.
 
My second question is:
What is this exerpt talking about, what am I expected to learn from it?
Because the heading says, "Schema Component Constraint", can this
exerpt be theoretical in some way?
 
Thank you
 
Regards
 
Rainer Becker