John Gregory Dunne seems to be (have been?) a reticent fellow, and out of his book I get no sense that he actually loves writing screenplays, or even that he loves writing novels and writes screenplays to support his novel writing habit.
But I was perversely gratified to read that Columbia Pictures tried to stiff him out of a delivery payment for a screenplay they didn't like, and that he had to go to the WGA, and wound up getting paid in full, with late charges. So those sorts of shenanigans, with which I am rather too familiar myself, go on even at the highest level.
My personal feeling, and the standard of the industry, enshrined in both the WGA and WGC contracts, is that if you don't like what a writer wrote, you still have to pay him -- he has, after all, done the work -- you simply don't work with him again, at least on that project. For some reason, producers who would never think they didn't owe an assistant director for his work, think they can reject a screenplay and not pay for the effort that went into it. (Of course producers also fail to pay assistant directors when they can get away with it, so you see the pattern. Maybe producers don't particularly think they don't owe the writer, they just think they can play on the writer's doubts about his own work to get away with not paying him.)