The fault I find with this reasoning is that it only works retroactively. The determination of whether or not this random woman committed a crime when she tweeted a rumor relies on the actions that other people decided to take.
You appear somehow ignorant how the law works. It is about adult humans being able to predict consequences of their actions.
If you are travelling at speed (but still below the speed limit) on an icy road and you kill someone, you go to prison for a long time as you should be able to predict you may kill someone.
If you shoot a projectile and it goes beyond the boundaries of your land, you may end up in jail again - you should be able to predict the projectile may go beyond the boundary.
She should have been able to predict the consequences of her spreading lies.
Adults are responsible for the consequences of their actions.