I can't see how you could be held responcible for another persons actions.
Neither do I.
If the ISP you use makes a mistake & causes a problem with OR`s equipment, then I`d expect the ISP to foot the bill or argue the toss with OR.
But I can see how it could get messy, OR pick the weakest link, the end user! Pay up or else.
It doesn`t seem right that an individual can take responsibility for an ISP`s cock-ups or possible negligence, it`d be interesting to see what a Court/Judge would say, after all we as consumers aren`t actually allowed to talk/communicate directly with OR, only an ISP can do this.
If OR were to try then I could see many simply paying, but I can see some telling them where to shove their invoice/charge. What happens next would make interesting news.