Well, then you completely misunderstanding things. It's not about joining, not at all. Actually what they ask for changes things for all employees of an industry (e.g. cars, or generally metal), not just for unionized workers.

So let me explain on this, as european union culture is VERY different from US union culture.

In Europe a lot of countries have a system of "Tarifvertrag" ("tarrif contract", but really "collective agreement"). In such contracts the workers union of a section (e.g. metal) do a contract with the business union of the employers. They settle then down to "give 28 days of holidays and x EUR per month to each" ... and then they won't strike for some time, e.g. 2 years. After this, they negotiate anew.

The special thing about this "Tarifvertrag" is now that EVERYBODY benefits from them, union member or not! In Sweden, for example, such general tariffs apply to 90% of the employees. So you can see how normal it's seen there. Anything that fights against this normality is thus seen not favorably (in Denmark it's 80% of all employees, in Germany it's much less, just 51% --- but still way more than in the US).

Now, Tesla Sweden says that they don't won't to follow such a model. The collective agreement is to them null and void. It seems that Elon Musky is, as many US brainwashed business people, strictly anti-union. Whatever comes from unions is surely coming right from hell :-) They have NEVER learned that with good unions you can have "peace at work", or more motivated people. The effect of this is visible, e.g. when you look at efficiency of e.g. german workers compared to their US counterparts. And this despite --- due to unions --- e.g. german cashiers can sit while checking people out. Horrors to US employee ... but effectively better for the people. So, in my view, US business people never learned how to use unions to their advantage --- which clearly is possible!

Now, with Musky's staunch "we ignore unions" stance, some reaction had to happen. Actually it was an escalation:

* in October 130 Tesla employees in various service stations went to strike

* in November 470 employees did strike

* now late December workers from other swedish unions said "We don't load/unload for Tesla in Sweden"

* and now, in early December, Danish and and Norwegian workers joined as well

Currently 12 different unions joined the "make Tesla think a bit more european" strike.