Even if you have a clear TOS, your still responsible for what your customers do with it. Check today's ruling that the RIAA won for instance, the proprietor of the usenet service was held responsible while using the Betamax defense, if your in a business agreement with customers then you have control over what they can and can't do. Missing action from your part to correct possible ongoing illegal activities could be seen the same way.
Just a thought, i'm not an expert in law.