One out of literally hundreds of millions of visitors. Many more have died on the highways driving to WDW. Should we shut them down too? if someone drowns in a tub do we remove the bathrooms?
Protecting people from extremely improbable risks is ridiculous. This, plus the useless security increase, the ugly gator fences, when will it end? It's chipping away at the enjoyment of visiting for no gains whatsoever.
It's also a liability issue. WDW's insurance is insane anyway but things like leaving a pool open and unattended all night would make the insurance rates even higher.
I believe that technically now all the leisure pools close - Disney has changed the rules in the past couple of years. I also don't care for this change.