As it is in Norway. The system is set up to be collectivist, and indeed it was largely due to the labour unions and labour parties that it became like this. I suppose Canada has something similar.
This kind of system implies that people are members of the group more than they are individuals, as least for some purposes. The union is allowed to make agreements on their behalf, the same way as a state may make laws on behalf of citizens. Often, this provides strength in numbers, but it may also limit bargaining power (and other freedoms) for individuals.
Now, if some group other than the offial union representatives start organizing workers independently of the union, that threatens the entire model. Not only does this cause and upredictable situation for the employer, but it undermines the power of the Union itself (and in particular the bosses).
This means that it's in the Union's interest if the agreements they have with the employer is legaly binding. A Union may even end up on the employers side in a conflict like this, to help identify ring leaders, bring forth secret messages that was passed to the users, etc.
Now for you Americans (US version), that lean left, but still don't get this: There are primarily two types of economic systems that have been introduced by "the left" as alternatives to capitalism.
One is the one described above, which is a sort of corporatism. This is the moderate version, where there is some room for individual freedom, but where some freedom is given up to unions, employers and the government (in exchange for security). This is the system that was introduced by Social Democrats in the mid 20th century, and you still find remains of it in traditional heavy industries, government employees, etc in Europe, though not to the same extent as in 1970.
The last one is full socialism, where the government generally takes over ALL control.