You absolutely have a point but I also think that you perhaps don't have the social or historical context to understand how deeply rooted unions are in Swedish society.

The Toysrus strike has been mentioned and at the time the unions did not even use all means at their disposal.

In the event of a general strike bus drivers could well refuse to stop at bus stops servicing the warehouse. Banks will refuse to process transactions for the company. Members will refuse to give them their business (4 million or so out of the total 10 million population). All of this happened to Toysrus.

Nearly 90% of workers in the country are members of a union and most will follow a general boycott if urged to do so.

There really isn't much of a choice here. They don't have to sign an agreement, but they certainly will not be profitable unless they do.