My ISP has a similar system. Except it works like this: if your machine is detected to be sending spam, for instance, the next time you try to view a webpage you're served an information page that your PC is compromised, please fix it and click here to not see this page again next time. Your actual traffic isn't compromised, it's just redirected to let you know of a problem. I can't tell you what would happen if you got close to your data limit, since we don't have any.