Safety for the persons at the event, to me(this is my personal opinion) seemed 2nd in priority on the part of the fair and concert officials. They should of played it safe and evac'd, the people out in the open especially. I know its easy to sit here and arm chair quarter back this, but at the first sign of a storm, especially having been informed of a Severe Storm warning, I would of without any doubt, ordered the Evac.
To a certain extent, you're probably right... "The show must go on" is truly a mindset in the industry. From the comfort and safety of my armchair, I'll disagree that the fans needed to be ordered to evac; they are free to do that at any time. An evac order probably would have helped the crew/employees as walking off the job is frowned upon... which makes me wonder where the steward for Local 30 was at... I haven't sorted this all the way through yet in my own mind, but outdoor gigs are significantly more dangerous than indoor gigs 99.9% of the time. Weather, terrain, improperly grounded gen-set's all conspire against you. Everything is more difficult when you have less control of the variables.