Unions have progressive disciplinary policies in place to fire workers that do not fulfill their expectations of employment. No union has a contract that allows that type of behaviour because the workers themselves would be negatively harmed by such unreliable behaviour.

You're bothered by the incompetence of your manager to go through the process laid out in the union's disciplinary policy and then blaming the entire union for it.

If the worker was actually that poor of a performer, the manager could have even gotten workers to establish a collective complaint against their colleague.

Your manager refuses to understand the rules and you suffered the consequences for it.

If you no longer worker for the union, perhaps you can tell me the union and the year so I can source their collective agreement for myself because I have never heard of a union that wouldn't at least have a discussion with that coworker.