You realize that historically labor unions predated government laws about labor unions, right?
But there were laws allowing freedom of association, so something like unions were default allowed, in the absence of any other laws.
There are many examples of this including Solidarity [1] and the Peasants' Revolt of 1381 [2] following attempts to freeze wages following the Black Death where demand suddenly exceeded supply and pushed up wages.
And this isn't even counting the cases where peasant and worker uprisings that led to revolutions.
The concept of a general strike is a relatively modern one but an extremely powerful one regardless of any legalities.
[1]: https://en.wikipedia.org/wiki/Solidarity_(Polish_trade_union...