In any kind of business relationship, there are two parties involved, usually a seller and a buyer. In this case, it's the business and a costumer. If a transaction is illegal, both are equally responsible and thus liable, not just one of them (e.g. rape is not a business relationship, so the victim cannot be blamed, but prostitution is (as long as it's not forced), so the prostitute is breaking the law as well).
If the US makes some business relationships illegal, when one of the parties is in the US, it should go after that one party, not after the other one. E.g. copying DVDs is illegal in the US, but legal in most of Europe. Should I be punished for offering the service of making backup copies of DVDs for EVERYONE, or should the US punish the US customer that was using a service that is illegal?
In a way, what the US did is disrespect and ignorance of Canadian national sovereignty. Canada seems to be OK with that, but I hope that other countries won't be...