Look at how many immigrants the United States has, a lot of American's come from another heritage. The only way terrorist's can use Canada as a safehaven is by immigrating there. If you are going to attack America, why would you immigrate to Canada and risk crossing the border when you can just directly immigrate to America?
America has terrorists in there country, just as Canada does. You don't see Canada complaining about America not being responsable enough to get rid of the terrorist cells in there own backyard and preaching about a risk of them crossing over and attacking Canada.
This is nothing more than more fear mongers, and using Canada as a escape goat... again.