“How could the organizers stop stupidity.”
This is exactly what safety regulations are for, the stupid ones. As I said, surprised it has not happened earlier.
Will be an interesting legal case.
Not really. Criminally, it will only be a question of what they charge her with if they choose to charge her at all. My guess is they won't charge her. Civilly, if any of the cyclists choose to sue her, her liability will be a given. The only questions will be the amount of their damages and where the money will come from. Most people have no liability insurance except for required car insurance, so unless this thirty-year old woman has some personal wealth, they're unlikely to see much of anything.
It happens every year to some extent. You would not have the tour as such a unique event without the fans as it is. They go past millions of fans every year without major mess ups like this. Watch it sometime and you will see.