Sure, but if they didn’t drain it some bloodsucking parasite (I mean a lawyer) would surely find some sick person who drank the water and file a lawsuit.
You are right on that part.
Alternatively, they could have pumped the water back to the filtration plant and 'refiltered' (or retreated) the water. Also they could have send it to a secondary water system for agriculture or ground watering purposes.
Draining it back into the local river system makes no sense at all.