All legislative items will deal with one single subject in the strictest interpitation.
Even with the detailed accountablity of roll call votes politicians have a nack for hiding their sneaky deeds within large bills.
By making bills deal with only one item (and maybe, no make that definatly, the financing) they can no longer hide things so easily. Also, it makes it much more obvious when they trade votes.
http://www.notesfromthefarm.com/article.php/20060512204101853