Apex why doesn't the standard controller set the current page when creating the controller (Unit Test)

In Apex Unit tests why doesn't the MyConrtoller myCont = new MyController(StandardContoller); call set the current page?

For instance if I have this page:

<apex:page standardController="DB_Object__c" extensions="MyExtension">
  <apex:form id="detail_list">
    <apex:detail />
    <apex:actionStatus id="readStatus">
    <apex:facet name="start">
        Loading, please wait...
   </apex:facet>
    <apex:facet name="stop"><apex:outputPanel >
        <apex:commandButton action="{!readData}"
            value="Update Data"
            rerender="detail_list"
            status="readStatus"/>
        {!remainingRecords}</apex:outputPanel>
    </apex:facet>
   </apex:actionStatus>
   </apex:form>
</apex:page> 

If my unit tests creates this:

DB_Object__c dbObj = new DB_Object__c();
dbObj.Name = 'test';
dbObj.Setting = 'aSetting';
insert dbObj;
Test.setCurrentPageReference(Page.Demo);
ApexPages.StandardController sc = new ApexPages.StandardController(dbObj);
MyExtension myExt = new MyExtension(sc);

Why does ApexPages.currentPage().getParameters().get('id'); fail? I have to do:

ApexPages.currentPage().getParameters().put('id',dbObj.id);

What is the point of passing in the dbObj to StandardController if it doesn't do anything with it? Is the intent that you send in a blank object and the extenstion uses this object? There doesn't seem to be a lot of documentation for StandardControllers and Unit Testing...

Thanks!