It's perfectly legal, and you can find information about it on the websites of the unions.
Anyway, here's the link: https://www.sjofolk.se/nyheter/2022/medlem-automatiskt/
Now you have the truth right in front of you. Use a translator if you don't understand it.
I'd like for you to apologize for slandering me here in the comments and repeatedly calling me a liar. Will you do that, or will you just down vote and move on, like a hacker?
It's just the difference between opt-in and opt-out and you can simply opt-out and anybody that is subject to these arrangements is aware of them. But then you also don't benefit from the union in times of strikes and such. With the very high fraction of employers that sign up for these programs it's pretty common to automatically enroll them in the union for that segment and usually - but probably not always - the HR department will ask you when they onboard you if you want to join the union or not.
Edit: Apologies for confusing you with the other poster.
Nobody has corrected me, it is a fact that some large employers in Sweden will sign you up to a union without your consent.
I should know, because it happened to me. Decades ago the company I was working for was sold to Sweden. One day I received a bill for a home insurance from a Swedish insurance company. I didn't own a home. Called the insurance company to ask what it was about. They apologized and said a Swedish union had signed me up with them. Called that union and they said yes now you're a member. Told them I didn't want to be a member. They told me I can't exit the union, my new employer had signed me up as a member, talk to them. I called HR and said that I'm not going to be a member of any union. They told me that it wasn't optional and I had to be a union member. I told them I'll get my lawyer if that's the case, and they relented and asked me to hand in a signed statement that I didn't wish to be a union member. And that was the end of it.
Then hackers here say that these things don't happen, even though the union clearly states on their webpage that this is standard procedure. And if it did happen, then it's a good thing. It is a bit funny, and dumb at the same time.
How would you feel if your company was sold to Utah. Then without you knowing, your new boss signs you up as a paying member of the Mormon church, and then the Mormon church signs you up for home insurance? It's kind of the same thing. The union - or the church - might be wonderful, but I'm not going to allow anybody to sign any contract in my name without my permission. That's fucked up.
What he said.
If that doesn't happens then they've made an error.
Your link also states that membership is entirely optional, and you can choose another union or none.
Finally, signing you up for expensive insurance absolutely requires your consent, as doing that is otherwise illegal. Your link also don't say anything about this, so no I don't have "this truth" in front of me.