Based on a True Story: Why Do We Take these BOATS?

Our entertainments have always incorporated our experiences: no art is effective or meaningful if it's completely alien. There's one genre whose truth claims attract and repel. In the past two weeks I've watched three movies which trumpet their connection to the news. ( 3 movies many questions )

I understand that memoirs, biographies, histories and non-fiction are fundamentally different from film. Are there particular economic forces which tie the initial version to the transformed work, even when they no longer match at all? What degree of conformity is required between "based on" or "inspired by" or "dramatized from" a true story?