What happens when a customer who really does have a modem that is vulnerable or outmoded runs into related issues? Is that customer going to accept "Well, we included it with our junk mail" as an explanation? As for email, does anyone use their ISP-provided email address anymore? Everyone has a third party provider (mostly Gmail).

I don't think there's any fault in logic in presuming that the best way to make sure a customer receives a notification is to insert as near to their known-active stream as possible. I don't condone altering that stream, but I think it would be nice if they could send a page, potentially at the browser or OS level, exclusive for system control and status messages (no sales, marketing, billing, or collection messages allowed).