But they aren't. Also unions can't push these sympathy strikes too far, by doing targeted denial of government services they are really making it easy for the current right wing government in Sweden to add regulations to prevent unions from denying government services via sympathy strikes in the future. There are already such laws for healthcare etc.
They aren't because the situation doesn't require it yet? If the rules/regulations regarding delivery of plates changes union action will change to address that. If the government does something to curb union power (in a non life and death situation) they'll likely lose the next election.