Because countries often have differing political and economic systems, agreements are needed to protect those invested in trade. Canada has signed numerous deals. Treaties, agreements and organizations to help settle disputes may be necessary, but they often favour the interests of business over citizens.
If the CNOOC/Nexen approval is given, before the Keystone pipeline is approved, a new set of questions for the Americans will be opened up. Letting China Inc. have special access could give President Obama a reason not to approve Keystone, and could also Romney, if he wins, a reason to consider not approving the line as he has pledged to do on Day One.
The perception is that Canada-U.S. relations are piling up with potential court cases and chilling diplomacy. Ignoring or bullying our closest friend and ally to improve our trade with China knowing their political environment and economic system seems quite risky. Does it worth failing our friendship with the U.S.?