Tesla workers don't want to strike so other workers are blocking them from work. That should be illegal.
However, because Tesla Sweden has repeatedly threatened repercussions (loss of company stock which is the main way Tesla Sweden matches market wages, loss of employment) if you strike, the picket lines are manned by other union members.
The unions are allowed to strike because Tesla refuse to sign the collective agreement that regulates the use of strikes while the employees in return are guaranteed a minimum wage, pensions and so on - as have been custom in Sweden over the last century.
Tesla could sign this agreement any time they want to, but they're choosing this fight while the unions wants regulation. That's on them, not anyone else - and Musk is perfectly free to take his business elsewhere if he don't want to conform to Swedish customs.
More than 90% of labor is covered by collective agreements in Sweden, and while the number of members has dropped over the last few decades, about 70% of the population is still members of a union. When Tesla comes in, breaking all unwritten rules and customs and consequently attacking the Swedish model - this is bound to meet heavy resistance from the Swedish workforce.
Can you imagine the outrage that would come from a Swedish company coming to the US, trying to change fundamental mechanics of the system there without the support of its citizens? Why shouldn't this scenario result in the same outrage and resistance here in Sweden?
The solution is simple, sign the agreement - as is custom here in Sweden. If they can't conform to Swedish customs then they are perfectly free to leave.
PS. I explained why it's legal and why other unions feel obligated to join in, but you have not provided any arguments to why it should be illegal. Please explain Russell's teapot in this context.
Tesla workers under IF Metall are under strike, there are scabs in any strike, Tesla was ferrying workers from other places to cross picket lines and do the work, that doesn't fly here in Sweden, hence the sympathy strikes by other unions to show Tesla that this attitude is despicable in this society.