I think it is a good idea to harvest rain water, but like what everybody has mentioned, the intended usage is quite crucial.

For example, my brother in law built a koi pond in his house. His roof has no gutter and he did not realise that most of the water from his roof flows down to where his koi pond is. He built it during the dry season and when the first bout of heavy rain came, almost all the fish died within half a day. He then monitored during the next heavy rain and realised a lot of rain water from the roof ended up in the pond due to the shape of the roof. After the rain, he measured the pond water with a PH tester and the acidity was off the charts for any koi to survive.

I guess if they live within the water basin which their treatment plant water is sourced from, there is some logic to such laws.