That is precisely the problem. If the notification were rendered as part of the page by the web server, no one would have issue with it (though it would likely

That is precisely the problem. If the notification were rendered as part of the page by the web server, no one would have issue with it (though it would likely be blocked by adblockers anyway). It's the fact that the ISP is modifying traffic in-route, to inject something that was never intended to be part of the page, that is the problem. I expect my ISP to be a neutral carrier of messages, not meddling and altering my mail to add whatever they happen to feel like adding today.