Sewage? Really?
There are plenty of reasons for swimming to not be allowed that have nothing to do with whether it's safe or advisable to be near it, use it to wet sand for sandcastles, or even dip one's toes in it. Have you seen the marketing photos of the resorts? Somebody posted one above with couple on the beach next to the Hobie Cat. Have you been on the beach at the Polynesian for fireworks?
And plenty of people eat the fish they catch, boat, and swim in lakes like that. In some areas, lakes like that are a source for public drinking water. It's not sewage.
I know Disney has created an image of hyper-sanitization for itself but it's perfectly reasonable for people to have a different take on the Disney lakes than yours.