A lot of movies purport to be "based on a true story," even when the finished product is highly fictionalized. Are there any rules or guidelines that govern the use of this label?
Not really. The "based on the true story…" tagline has become something of a cliché for television movies-of-the-week, along with its insidious variants: "inspired by…", "in the vein of…" and the rest. You’re right in assuming that the phrase means almost nothing anymore.
I suppose a very bored, very litigious television viewer could sue a television network claiming false advertising if the movie was really nothing like the "actual events" it was based on, but what are the damages, really? Two hours wasted?
The only person who could legitimately claim damages is one of the "real people" portrayed in the movie, under libel law. That’s why a network legal department is careful to check out both the script and the marketing to make sure that none of the portrayals could bring on a lawsuit.