Actually basic cable here is $19 or $20 a month so YES they are losing money if the sub JUST has internet and gets basic without paying. That's at least $5 a month they are losing. I would be like buying dry-loop dsl and getting local phone with it for free. Isn't going to happen.
Although it depends on the franchise, basic cable is usually $13, while internet is $15 more without cable TV, and Comcast has to pay for the content for basic cable, they don't have to pay if it's accidentally on the line.