You've hit the nail on the head. I've seen both sides of the union coin. Unions need competition just as much as business do to function properly.

My father is a trades person and when one union treated him badly he left for another one cause he had options. His union has also saved him from a lot of BS that companies he's worked for have tried to pull.

Alternatively when I was a student I worked a part time job where I was forced to be a part of a union with no options. Ever week they took money from my paycheck and the one time I had a grievance they pretty much ignored me. Making sure the girl that showed up hung over and vomiting every 2nd shift was not fired was more important because seniority.

In Norway, each employee freely chooses their union. It is common in some industries that people belong to several different unions, and that some are not part of union. And usually they are based on particular profession, like engineering or economics. Unions then have more a passive role, offering services like free legal representation, better insurance deals, and training.

Only some areas like teachers seem to have near-homogeneous membership and unions taking an active role. And this seems mostly to be because 90% have the same employeer - the state (public education). Because of standardized working terms (incl salary), they have to be negotiated centrally.

Why does there have to be standard working terms? In the US professional baseball, hockey, and football have player unions and things like salary are negotiated per person.

They aren't standard working terms because of the unions. They are standard working terms because all teachers (who work in state schools at least) are employed by the local education authorities, who in turn set their working terms and salaries based on the government's department for education. So for teachers there is no individual salary negotiation, it's all decided centrally by the government and you either take it, or move to the private schools of which there are a lot less. (This is UK for me, but based on what op said, I think it's similar)

Yep very similar. There is built in some automatic compensation for years of service, and the payscale is inflation adjusted. For a given position, there is a small range of possible salaries. So it is in principle possible to have a bit of independently negotiated salary. In practice almost everyone falls very close to each-other.

This is not to say that individual teachers cannot earn more or less, just that the scale on which they are being paid has been decided in advance, right?

Percentage of revenue going to the players has already been negotiated.

It should also be noted that the players union represents a tiny portion of the actual employees involved and they actively negotiate money away from other employees.