PermissionSet.Demand does NOT throw Exception in HttpHandler in medium trust level with framework 2.0&3.5

Hi,
I have a class library with only one IHttphandler implementation, I implement ProcessRequest with following code in order to check whether web site is in medium trust level,
public void ProcessRequest(HttpContext context)
{
    new PermissionSet(PermissionState.Unrestricted).Demand();
}
Then I install this assembly to GAC, and create a new WebSite, set trust level to medium and framework to 2.0&3.5.  Then browse the web site, I found the Demand() method does NOT throw SecurityException.  But it does in framework 4.0.
If I put this code in Page_Load, it does throw SecurityException.  What happens to HttpHandler with FW 2.0&3.5?
Thanks,
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