fieldset element at XHTML Print 1.0

In XHTML Print 1.0, elements like fieldset, legend and optgroup are
not allowed. Steven kindly told me that XHTML Print 1.0 uses the Basic
Forms module from XHTML Modularization (which does not recognize such
elements). Can someone explain why these elements were rejected from
the Basic Forms module from XHTML Modularization? Wouldn't it be wiser to 
have fieldset, legend and optgroup allowed in the Basic Forms module?

Thanks.

Vicente Luque Centeno
Dep. Ingeniería Telemática
Universidad Carlos III de Madrid
http://www.it.uc3m.es/vlc