Over here, your water bill has 2 components, the price of the fresh water and the price of the waste water. Waste water is at least as expensive per m³ as fresh water, sometimes more than double. If you are using rain water, you save on the fresh water price, but you have to still pay for the waste water. And since waste water price is calculated from your fresh water consumption, you either have to have a second meter for the rain water (expensive) or you pay for an estimated amount of rainwater calculated from the roof area you are collecting from. Also very expensive, because the estimate is always not in your favour.
So not really "free" at all, and rather expensive enough that nobody does it...