It’s a strike. How could the workers be prohibited from deciding whether to strike?
The thing is that government services must be provided to everyone or (if enough people are on strike) not at all.
In a way the Swedish model has found an equilibrium between company powers and workers right that has been very productive.
Until an ignorant American enters the market and think he knows better than everyone else
Prevented, though, is harder. Prohibitions aren't always effective prevention.
Not everyone has state postal services like in the US
If all sympathy strikes were to be total walkouts then it would result in a general strike and that doesn’t target any specific party and thus doesn’t put pressure on them