The link says - thanks, Peace - that soft water has been treated, implying that it was purposefully done but why is rain water so soft?

What is 'sweet' water? Alkaline or acid? I know that lime is added to soil to balance its ph but is that called 'sweetening' it?

See, I'm still uninformed and uneducated. It appears - thanks, Amos - that acid and alkaline are not the terms to use.