Earlier quoted context omitted.
You'll have to link to how they mention its fine for the employer to force sign you without your consent, and to sign you up for expensive insurance again without your consent.
You could just have found the information yourself, it's not difficult 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.