An ISP contract cannot make you legally liable to other people's actions that have nothing to do with the ISP themselves. You can't ask for more volume based off "other people used it, not me"; but they can't say "if you don't secure your wireless you are legally responsible for BP's oil spills".
As an ISP is nothing but a carrier (this is why net neutrality is important), they cannot make you responsible for copyright infringements other people commit on your open network.
*if* the ISP is not liable for those copyright infringements otherwise; and *if* the law doesn't make you liable already.
Then again, IANAL; there may be backdoors that prevent common sense from being useful. Come to think of it, there probably are.