As you agree with Bonzo, pfr please tell me why YOU specifically:

".... feel disappointed, even cheated, when I've enjoyed a tremendously moving film "Based on a true story / Inspired by true events", and I discover the entire story is riddled with significant characters who were invented and never existed, and plot developments and dramatic situations that are completely fabricated..."

Anything to do with the fact that anything that has been shot, or written, retrospectively, where all facts are known has a duty to represent the event truthfully? If it doesn't then that is how myths and lies are reinforced and propagated.