It isn't that hard to see why they haven't redirected users
There are two glaring reasons that they haven't redirected users.
1. The users would ignore the messages. Users have finally started to listen (at least some) when we tell them not to believe a pop-up or email. You really want us to have to tell them that this time it is real?
2. The legalities of hijacking a user's traffic are pretty clear in that it is illegal. Just acting as a proxy required a court order.
In the end, just cutting the existing computers off fulfills everything that all the suggestions above look to achieve with the major plus of less legal wrangling and red tape.