That seems an odd thing to say, given that basically all you need is software to drive your cable modem...
... I guess they mean they won't offer support for Mac users on OS X. In that case, stuff 'em!
There are plenty of other broadband ISPs out there...
BT Openworld only offer "all-in" ADSL solutions at the moment, despite trialling "DIY" (i.e. "wires only") ADSL solutions ages ago. By this I mean that they will come to your home and install ADSL for you, including the provision of a simple USB DSL modem.
However, their wholesale wires-only business kicked off a while back (you provide your own modem / router, which is much better, as you can link your whole network to ADSL that way), so you'll find there are plenty of ADSL broadband providers out there. Check out
http://www.adslguide.org.uk for the low-down of ISPs and how to get up and running -- they even have a Mac forum on there, and there are plenty of people using wires-only or BT-provided ADSL solutions with OS X.
I'm assuming your area caters for ADSL use of course.
My area is ADSL-enabled, but no dice with NTL / TeleWest... who keep mailing me about their broadband solutions anyway. Go figure! I don't have ADSL, so can't tell you much more. Just been looking into it as an alternative to commuting to work every day, given how crap trains are in the UK.
HTH!