Not all authentication schemes support password policies. If the authentication scheme does not support Password Policies, the check box description is dimmed and the check box is unavailable.
Following authentication schemes will have checkbox "Paswords Policies Enabled for this authentication scheme dimmed and unavailable for selection:
For windows authentication scheme, even though the checkbox is available, some functionality will not be available. With IWA, password is not available to SiteMinder at the time to authentication since IIS server does the authentication together with the browser and the AD.