Because the cable companies don't have to pay on their own channel's. Its a bit pointless spending money advertising to an audience who would have to spend lots of money just to get on to cable rather than to target an audience they already know has cable.
i think that was misunderstood, example, ntl advertise their company as in, hey every1 buy ntl, but y do it to ppl who already have ntl? does that not defy the point of doin it?
Having worked on ntl:home's advertising, I can shed some light on this subject. For TV advertising cable companies only tend to promote servies like 'pay-per-view' (in ntl's case it is called 'front:row') and 'recommend-a-friend' promotions. When trying to acquire 'new' customers they spend the majority of their budgets on a combination of Direct Mail, Press Adverts and Outdoor Advertising.