How is it extortion, legal or otherwise?

I genuinely don't know why employers do not hire non-union employees in such circumstances; I think most of us assume they "can't", but would like to hear from someone who actually knows.

Collective bargaining is pretty much the only tool workers have to even begin to correct the power differential between employer and employee. Ideally, it allows employees and employers to negotiate on more equal terms than is otherwise possible.

No, it isn’t. There is the individual threat of quitting. And there are the courts.

1. An individual quitting at a company where they are one single cog doesn't impact the company in any measurable way.

2. The court system in the west gives an asymmetric level of power toward the corporation, an individual has neither the time, the money, or the ability to navigate an ongoing protracted lawsuit with a corp that dwarfs them in all three aspects.

I've personally used both to extract concessions from employers.

In any case, my general point is that we need more protections in law for workers. Unions are a good way to distract from that.

Whereas the employee is free to go to work for some other company at any time.

The rules are not in balance.

> Whereas the employee is free to go to work for some other company at any time.

And generally the employer is free to fire people at any time. And to forbid employees from working elsewhere.

I don't think your examples hold up to scrutiny.

> And to forbid employees from working elsewhere

That's simply not true.

Who said anything about that? Scab labor is an extremely common thing. How you come to the conclusion that "the company is not allowed to hire other people to replace the striking workers", I'm entirely unsure. For two very popular examples, both in the air travel industry, both pilot's strikes and ATC strikes have had "other people hired to replace striking workers", with varying degrees of notoriety (being at Boeing, I can't imagine you being entirely unaware of either).

> That's simply not true.

Many employment contracts have "no moonlighting" clauses. Only two states specifically forbid them: Washington, and Washington DC (and Washington's policy only affects lower-paid workers) (https://sbshrs.adpinfo.com/blog/7-faqs-about-moonlighting-po... and https://www.rocketlawyer.com/business-and-contracts/employer...)

The ATC was a special case. It was illegal for them to strike, they struck anyway, so Reagan fired them.

"It shall be unlawful for any employer willingly and knowingly to utilize any professional strikebreaker to replace an employee or employees involved in a strike or lockout at a place of business located within this state. 1134.2."

Which is to say, most ongoing services in a capitalist country are provided under the “threat” that if you break the deal you agreed to, you’ll have to look elsewhere to get that service going forward.