Despite politically-charged rhetoric, juries in the United States tend to be very reasonable, and to demonstrate a highly sensitive B.S. meter for crazy arguments. Juries tend to be very lenient on defendants in situations where people got hurt in random, unforeseen ways. What riles juries is corporate arrogance -- managers who ignore *repeated* warnings about a dangerous problem, usually out of a belief that no one, either the government or a jury, will make them pay if something goes wrong. That's what got McDonald's nailed in the infamous spilled coffee case and that's what appears to have cost Six Flags here.
Also, I wonder if this is in some way an indictment to Six Flags for attracting hooligan crowds. That's been a major problem with them for years.
No, Six Flags need not ban rocks. It simply needs to put GRASS under its coasters, especially when the company has been warned that people are lobbing rocks from underneath a coaster up on to its tracks. That's just common sense. And when a business ignores common sense and that results in someone's injury, a jury is gonna make 'em pay. As it should.