Re: @J.G.Harston - again
You may log on to a system, but there is a HUGE difference between a system and the network, and I say again that if you do not understand the difference, you should not be commenting on stories like this.
You really don't log in to a home network, not unless you have implemented domain level accounts and an authentication server, in which case you are really logging into the domain. I strongly suspect that you haven't, although I do admit the possibility.
On all Windows systems I've administered outside a company environment, the network settings are set up on a per system basis, not a per account basis. This means that once logged in to a system with any account, all network access is the same. And it is normally not possible for a web site to know what user account is in use on a particular PC (that's why they go to so much trouble putting cookies in your cache, so they can track who wou are). So to the ISPs web site that the popup comes from, there is no way of knowing whether the account is Tarquin's or Dad's. That level of information is just not available to the web site.
What the ISPs may end up doing is directing you to a site where you have to log in to the web site, using an account that was set up when the account was set up. This would do what they need, but would render the entire home network unusable until the account owner was available. And I suspect that many users (like me) do not use that account, so may not remember the user id and password for that site.
I suspect that I have been locking down my Windows PCs so that most users are not using Admin for longer than you. My background is 30+ years of administering UNIX systems, so privilege separation is engrained in my psyche, and I learned how to do it for my PCs (together with a mechanism of relaxing it for those STUPID programs that need admin rights) almost as soon as I got an NT based system in the house, which was after I started putting Linux on all my PCs.