Most of us have probably seen the ad for Brita water filters which basically tells us that the water we get out of our kitchen sinks is the same water we use in our toilets, therefore, we should filter our water that comes from our kitchens.
I don't think the question should be why are we drinking toilet water. The question should be: "Why we use tap water to piss in?"
I'd like to learn more about greywater systems. This is where waste water from sinks and showers are used for toilet water and irrigation. I've heard that it's not a difficult plumbing job, but just takes a little out-of-the-box thinking.
Drink our tapwater. There's nothing wrong with it. It's free, it's clean, and and it's available. Not everyone in the world has that kind of luck. Let's not take for granted the most basic human needs which are simply handed to us.