I wouldn't blame the ravers for their eyes getting burnt, the idiots putting on the show should have safe guarded and either used a safe form of light show, or not do it at all.
However, if anybody at this rave was doing any drugs such as E, chances are they're vision is royally screwed, since E makes your pupils dialate and absorb more light information then normal.
As it goes for waiting a few days to goto the hospital, if it's anything like where I live for waiting times, chances are they might have figured it would have been temporary and didn't want to wait hours in an emergency room to hear the doctor to tell them to go home.
Either that or they wanted to wait it out until the drugs were flushed out of their system before they went to the hospital.