There are people working for very little, but it is very rare. For example you make at least $15 an hour at McDonalds in Sweden since that is what the union agreement is for that sector. McDonalds isn't forced to sign it, but if they do they are protected against strikes. However you can have a small company without union agreement and pay whatever you like, but if your employees are in a union they might use it to start a strike action if your conditions are too bad.

Note that in Sweden unions aren't company specific so there is no way to prevent people from joining one.

Sometimes it's not about what the law says, but about what society does. In practice, these are the minimum wages. Perhaps you can diverge from them for a while without punishment, but that doesn't make them not the minimum wages. It just means that the enforcement is different.

Arguing about the legality is a harmful modern fetish that especially in contexts such as these does not help us move along.

It would be a civil breach of contract between the employer and the union, right?

Not for those who aren’t part of a union. I’m asking to understand the edge case, not to see if an employer could violate a union contract.