Flushing Our Water Down the Drain

In my travels to South Africa, Zambia, and Botswana last summer, I noticed that the toilets typically had two buttons for flushing.  One with a half-circle and one with a complete circle.  The half-circle button is for flushing after you pee, so it uses very little water.  The full circle is only used when a full flush is advisable.

So simple, and yet so effective.  Why don't we do that here is the USA (and everywhere)?