I used to be a Certified Swimming Pool Operator, something that was required by the state for taking care of pools used by the public. In this case it was for several apartment swimming pools.Cryptosporidium is one of a handful of parasites/bacteria that normal levels of most disinfectant agents such as sodium hypochlorite will not kill. If someone comes to the park with this parasite in their system there's little that a properly chlorinated pool would do to it. If there's even a suspicion of it they usually close the affected pool or park and then super-chlorinate the water which will kill the bacteria at that level. After 24 hours or so the chlorine levels return to a safe level and the pool can reopen.
Six Flags probably did nothing wrong but people will still point fingers, that's their nature I guess.