Why not? China just detained somebody for no reason, to unknown location, without any stated charge, with no arrest warrants, no hearing, not allowing him to legal counsel, no consular access. China is violating his basic human rights in every single way. If they can do that to a Canadian, besides American, they can do this to anybody as long as they are not pleased with something. So what are you going to do from now on? Walking on tippy toe with China and say goodbye to our freedom and values and our laws and rules? Because anything you say or do, you can risk having your own country's people getting kidnapped. Today China is punishing Canada because Canada helped US, so tomorrow what happens when France did something that China didn't like? Is France going to have to worry about any of its French citizens in China disappearing for no reason?
China is not doing this to target Canada anymore; it's sending a message to the whole world saying "I can bully you anytime doing whatever I want whenever I am pissed so forget about your democracy or freedom or values or laws, it's my way or the highway. You better do what I say or your people gets it. Canada is an example." The world would not side with USA when China is getting rich making money beating USA in technology but the world will side with USA if China is bullying an innocent third-party country.
Actually China is sending a message - if you arrest our well-known citizens on trumped up bullshiat we can also arrest your well-known citizens on trumped up bullshiat.