I'm not sure how familiar you are with employment laws in Sweden, but you cannot just fire people willy-nilly, you need to have "Objective grounds for dismissal" in order to fire someone. For example, you are not allowed to fire someone because they're participating in a union, as a employer you'd be setting up yourself to get sued if you do this.

Question is, what "participating" actually refers to. One could make the case that striking is just a part of normal participation in the union, while one could also make the opposite case of course. In the end, seems it'd be up to the courts to decide.