This is probably part of their "Web Notifcation System". They have a published RFC talking about how it works (RFC6108).
Using that system they can selectively notify customers. Like if they detect your system is infected with a virus. Or warn you your service will be discontinued if you don't pay your bill.
Look at all the work that went into that RFC. Unbelievable that they couldn't get a half-decent developer to verify that the notification is coded well enough to even show properly.
I agree. The entire concept is about trying to be less invasive in the web browsing experience (by adding a popup instead of redirect the entire web session) but that all falls apart because of crappy JavaScript.
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.
Using that system they can selectively notify customers. Like if they detect your system is infected with a virus. Or warn you your service will be discontinued if you don't pay your bill.
http://tools.ietf.org/html/rfc6108