Because border guards have been abusing their authority to deny entrance to anyone they don't want to regardless of reason. There have been numerous cases cited of Canadian citizens with proper documentation being denied entrance to the US by border officials because of their last names or appearance. I know of a family of East Indian ethnicity who were all born in Canada who were hassled at the border despite having Nexus passes just because the border official didn't like their last name and was on a power trip. They weren't Muslim nor did they originate from one of the banned countries on the list. Didn't stop the border official from harassing them.

The TDSB has a large number of Muslim students, that all though of Canadian citizenship or landed immigrant status, could still be barred from entering the US at the discretion of border officials.