The simple answer is that as long as the Canadian remote worker is physically performing the work in Canada, no US work visa is required.
However, if at some point your Canadian employee needs to visit the US for work purposes, they will need some type of visa to enter and stay in the US. This will depend mostly on the amount of time they will spend and the type of activity.
不知道上面的解释是否权威, 貌似如果不进入US的话,可以不用办理TN visa。