Why can't I have a cable card with limited cable?

I have a limited cable through Comcast and TIVO HD. I would like to record the HD channels that I get with limited cable, but Comcast will not give me a cablecard (the person at Comcast could not give me a reason for this policy, so I am assuming it is designed to force me to pay an extra $40 a month for a service I don't want or need). Will I be able to use my TIVO HD once the digital conversion happens and is there any way to get a cable card with limited cable?