A nice thought, but I don't think it will ever fly. BT's argument is that it provides the telephone lines; hown you use them is your business. If your argument was to hold, then the Post Office would be responsible for all the junk mail you receive and should be prosecuted if it delivers anything pornographic, even if you ordered it.
Every individual is responsible for their own actions and answerable for their lack of actions. Why should BT provide firewalls, virus scanning, spam filtering etc, especially if they were not the ISP?