when two terror attacks were success against two Canadian soldiers. I was critical for similar reasons, except in Canada there were noticeable differences. In both cases here the suspects were on the radar to such a degree that in one case the RCMP were actually in the process of watching him for hours from their car...this terrorist ran over a soldier in front of their eyes. In the second case he had been on the radar and had attended a mosque in Alberta that had a history of radicalism, he shot and killed a Canadian soldier on Parliament Hill.
My criticism of CSIS and the RCMP is in regards to how they generate business for themselves, often wasting time "chasing rabbits" and citizens who have never been threats. Low-hanging fruit with little support in fact but rather loose information and even self serving agents (google Maher Arar and you can read first hand how the RCMP passes on erroneous intel even to the U.S). I don't think the FBI have the luxury to waste resources on insincere cases as the U.S have enough legitimate and real concerns.
Ultimately you cannot stop every incident. This was a particularly difficult attack to have spotted (outside of the tweet the terrorists made) due to the proximity (they drove from Arizona), and the broad number that were probably offended by the event itself.
I am fully supportive of free speech, certainly with these types of events it can be a logistical nightmare. The lady who organized this event spent thousands on extra security alone. She might be an unpopular person, certainly these types of events wouldn't interest me, but noone has a right to respond to an offensive expression with murder, no matter the degree of provocation.