It helps to know the law.
1. The individual had no criminal record in the U.S. or anywhere else.
2. Under the Safe Third Country Act, he could not file for refugee status  if he had no family in Canada, OR if he had not already been ordered deported from the US after applying for Asylum there.   
On point 2, before everyone screams that this provision is bad, it was put in place to protect refugees from certain countries who the U.S. does not agree are really refugees, like El Salvador and Guatemala, where the US government, for its own reasons, supports dictatorships that employ death squads.   El Salvadoran refugees are particularly at risk, because the US has rarely accepted any of them as refugees.   No objective person would ever agree with this. 
So one person who came to Canada that way 5 years ago became radicalized and committed a terrible act (allegedly), and we should therefore close the door to thousands of refugees who really need our help?
The CBSA officers did their job.