But you're pointing to someone at the end of a long and successful career. Of course he gets to dictate terms. He's rich and famous! Look at the Wiki description of his early career:
In 1952, Williams was drafted into the U.S. Air Force, where he played the piano, brass and conducted and arranged music for The U.S. Air Force Band as part of his assignments. In a 2016 interview with the US Air Force band, he recounted having attended basic Air Force training at Lackland base (San Antonio, Texas), after which he served as a pianist and brass player, with secondary duties of making arrangements for three years. He also attended music courses at the University of Arizona as part of his service . . . .
Known as "Johnny" during the 1950s and early 1960s, Williams composed the music for many television programs (including several episodes of M Squad), and served as music arranger and bandleader for a series of popular music albums with the singer Frankie Laine . . . .
After his studies at Juilliard and the Eastman School of Music, Williams returned to Los Angeles, where he began working as an orchestrator at film studios . . . .
Williams was also a studio pianist, performing on film scores by composers such as Jerry Goldsmith, Elmer Bernstein, and Henry Mancini.
https://en.wikipedia.org/wiki/John_Williams#Early_life_and_family
Do you really think John Williams was in a position to dictate the terms of his work, to say what he would or wouldn't do,
in the Air Force? People have been throwing the term "good soldier" around here, a bit disparagingly I might add, but John Williams
literally got his start as a professional musician in the military. And then he worked for other people for the better part of a decade.
Same thing with Bernard Herrman, who started out as a
staff conductor at CBS, and Goldsmith, who
started in the CBS music department a couple of decades later. I mean, these guys came up in the
studio system. You really think they weren't pushed around by directors and department heads and studio heads? Everybody pays their dues. You get to make demands and be temperamental
later, when (if you're lucky) you're hot enough to be valuable in the market despite your quirks, temper, etc.
And Herrmann wasn't "betrayed" by Hitchcock. They had a disagreement, and he had less authority than Hitchcock, so he lost. Herrmann had the freedom to stand on his principles, but only because he was a highly successful, known quantity. He knew he could make money elsewhere. What you're celebrating isn't the greatness or personal independence of the artist -- it's his market value. It's basically the equivalent of
Christian Bale yelling at a cameraman.
It's great that some talented artists (eventually) attain that kind of clout and freedom. Good for them. But it's not something people have at the start of their careers; it's something you
attain.